Image-forming substrate and image-forming apparatus using same

ABSTRACT

An image-forming substrate has a base member, and a layer of microcapsules coated over the base member. The microcapsule layer contains a first type of microcapsule filled with a first type of single-color dye, and a second type of microcapsule filled with a second type of single-color dye. The first and second types of single-color dyes comprise a same single-color dye exhibiting differing densities. The first type of microcapsule exhibits a first characteristic such that, when the first type of microcapsule is squashed under a first pressure at a first temperature, discharge of the first type of single-color dye from the squashed microcapsule occurs. The second type of microcapsule exhibits a second characteristic such that, when the second type of microcapsule is squashed under the first pressure at a second temperature, discharge of the second type of single-color dye from the squashed microcapsule occurs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image-forming substrate coated witha layer of microcapsules filled with dye or ink, on which an image isformed by selectively squashing or breaking microcapsules in the layerof microcapsules, and also relates an image-forming apparatus using suchan image-forming substrate.

2. Description of the Related Art

In a conventional type of image-forming substrate coated with a layer ofmicrocapsules filled with dye or ink, a shell of each microcapsule isformed from a suitable photo-setting resin, and an optical image isrecorded and formed as a latent image on the layer of microcapsules byexposing it to light rays in accordance with image-pixel signals. Then,the latent image is developed by exerting a pressure on the layer ofmicrocapsules. Namely, the microcapsules, which are not exposed to thelight rays, are broken, whereby the dye or ink is discharged from thebroken microcapsules, and thus the latent image is visually developed bythe discharging of the dye or ink.

The formation of the image on the layer of microcapsules is performed byproducing image-pixel dots in accordance with image-pixel signals. Eachof the image-pixel dots has a larger diameter than that of themicrocapsules, and thus plural microcapsules are included in eachimage-pixel dot area. Each of the image-pixel dots is developed orcolored by breaking the microcapsules included in the correspondingimage-pixel dot area, and the colored image-pixel dot merely exhibits agiven constant density. Namely, it is unknown to vary a density of thecolored image-pixel dot per se.

Also, each of the conventional image-forming substrates must be packedso as to be protected from being exposed to light, resulting in awastage of materials. Further, the image-forming substrates must behandled such that they are not subjected to excess pressure due to thesoftness of unexposed microcapsules, resulting in an undesireddischarging of the dye or ink.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide animage-forming substrate coated with a layer of microcapsules filled withdye or ink, wherein it is possible to vary a density of an image-pixeldot per se, which is to be produced and colored on the image-formingsubstrate by selectively squashing and breaking the microcapsulesincluded in a corresponding image-dot area.

Also, another object of the present invention is to provide animage-forming apparatus using such an image-forming substrate.

In accordance with a first aspect of the present invention, there isprovided an image-forming substrate which comprises a base member, and alayer of microcapsules, coated over the base member, containing a firsttype of microcapsule filled with a first type of first-single-color dye,and a second type of microcapsule filled with a second type offirst-single-color dye. The first type of microcapsule exhibits a firsttemperature/pressure characteristic such that, when the first type ofmicrocapsule is squashed under a first predetermined pressure at a firstpredetermined temperature, discharge of the first type offirst-single-color dye from the squashed microcapsule occurs. The secondtype of microcapsule exhibits a second temperature/pressurecharacteristic such that, when the second type of microcapsule issquashed under the first predetermined pressure at a secondpredetermined temperature, discharge of the second type offirst-single-color dye from the squashed microcapsule occurs.

According to the first aspect of the present invention, to form an imageon the image-forming substrate featuring the first and second types ofmicrocapsules, there is provided an image-forming apparatus whichcomprises a pressure applicator that exerts the first predeterminedpressure on the image-forming substrate, and a thermal heater thatselectively heats a localized area, on which the first predeterminedpressure is exerted by the first pressure applicator, to one of thefirst predetermined temperature and the second predetermined temperaturein accordance with image-pixel information carrying gradationinformation.

In the first aspect of the present invention, the layer of microcapsulesalso may comprise a third type of microcapsule filled with a first typeof second-single-color dye, and a fourth type of microcapsule filledwith a second type of second-single-color dye. The third type ofmicrocapsule exhibits a third temperature/pressure characteristic suchthat, when the third type of microcapsule is squashed under a secondpredetermined pressure at a third predetermined temperature, dischargeof the first type of second-single-color dye from the squashedmicrocapsule occurs. The fourth type of microcapsule exhibits a fourthtemperature/pressure characteristic such that, when the fourth type ofmicrocapsule is squashed under the second predetermined pressure at afourth predetermined temperature, discharge of the second type ofsecond-single-color dye from the squashed microcapsule occurs.

According to the first aspect of the present invention, to form an imageon the image-forming substrate featuring the first, second, third andfourth types of microcapsules, there is provided an image-formingapparatus which comprises a first pressure applicator that exerts thefirst predetermined pressure on the image-forming substrate, a secondpressure applicator that exerts the second predetermined pressure on theimage-forming substrate, a first thermal heater that selectively heats alocalized area, on which the first predetermined pressure is exerted bythe first pressure applicator, to one of the first predeterminedtemperature and the second predetermined temperature in accordance withfirst-single-color image-pixel information carrying gradationinformation, and a second thermal heater that selectively heats alocalized area, on which the second predetermined pressure is exerted bythe second pressure applicator, to one of the third predeterminedtemperature and the fourth predetermined temperature in accordance withsecond-single-color image-pixel information carrying gradationinformation.

In the first aspect of the present invention, the layer of microcapsulesmay further comprise a fifth type of microcapsule filled with a firsttype of third-single-color dye, and a sixth type of microcapsule filledwith a second type of third-single-color dye. The fifth type ofmicrocapsule exhibits a fifth temperature/pressure characteristic suchthat, when the fifth type of microcapsule is squashed under a thirdpredetermined pressure at a fifth predetermined temperature, dischargeof the first type of third-single-color dye from the squashedmicrocapsule occurs. The sixth type of microcapsule exhibits a sixthtemperature/pressure characteristic such that, when the sixth type ofmicrocapsule is squashed under the third predetermined pressure at asixth predetermined temperature, discharge of the second type ofthird-single-color dye from the squashed microcapsule occurs.

According to the first aspect of the present invention, to form an imageon the image-forming substrate featuring the first, second, third,fourth, fifth and sixth types of microcapsules, there is provided animage-forming apparatus which comprises a first pressure applicator thatexerts the first predetermined pressure on the image-forming substrate,a second pressure applicator that exerts the second predeterminedpressure on the image-forming substrate, a third pressure applicatorthat exerts the third predetermined pressure on the image-formingsubstrate, a first thermal heater that selectively heats a localizedarea, on which the first predetermined pressure is exerted by the firstpressure applicator, to one of the first predetermined temperature andthe second predetermined temperature in accordance withfirst-single-color image-pixel information carrying gradationinformation, a second thermal heater that selectively heats a localizedarea, on which the second predetermined pressure is exerted by thesecond pressure applicator, to one of the third predeterminedtemperature and the fourth predetermined temperature in accordance withsecond-single-color image-pixel information carrying gradationinformation; and a third thermal heater that selectively heats alocalized area, on which the third predetermined pressure is exerted bythe third pressure applicator, to one of the fifth predeterminedtemperature and the sixth predetermined temperature in accordance withthird-single-color image-pixel information carrying gradationinformation.

In the first aspect of the present invention, the first and second typesof first-single-color dyes may exhibit the same density or may exhibitdifferent densities; the first and second types of second-single-colordyes may exhibit the same density or may exhibit different densities;and the first and second types of third-single-color dyes may exhibitthe same density or may exhibit different densities. If necessary, thefirst-single-color dye, the second-single-color dye and thethird-single-color dye may comprise a same single-color dye exhibitingdiffering densities.

Preferably, the first-single-color dye, the second-single-color dye andthe third-single-color dye may comprise three-primary color dyes. Inthis case, the image-forming substrate may further comprise anadditional layer of microcapsules filled with black dye coated over thelayer of microcapsules, and the microcapsules, included in theadditional layer of microcapsules, is formed of resin such that they areat least thermally plasticized at a greater temperature than the sixthpredetermined temperature and under a lower pressure than the thirdpredetermined pressure.

In accordance with a second aspect of the present invention, there isprovided an image-forming substrate which comprises a base member, and alayer of microcapsules, coated over the base member, containing a firsttype of microcapsule filled with a first type of first-single-color dye,and a second type of microcapsule filled with a first type offirst-single-color dye. The first type of microcapsule exhibits a firsttemperature/pressure characteristic such that, when the first type ofmicrocapsule is squashed under a first predetermined pressure at a firstpredetermined temperature, discharge of the first type offirst-single-color dye from the squashed microcapsule occurs. The secondtype of microcapsule exhibits a second temperature/pressurecharacteristic such that, when the second type of microcapsule issquashed under a second predetermined pressure at a second predeterminedtemperature, discharge of the second type of first-single-color dye fromthe squashed microcapsule occurs.

According to the second aspect of the present invention, to form animage on the image-forming substrate featuring the first and secondtypes of microcapsules, there is provided an image-forming apparatuswhich comprises a first pressure applicator that exerts the firstpredetermined pressure on the image-forming substrate, a second pressureapplicator that exerts the second predetermined pressure on theimage-forming substrate, a first thermal heater that selectively heats alocalized area, on which the first predetermined pressure is exerted bythe first pressure applicator, to the first predetermined temperature inaccordance with first-single-color image-pixel information carryinggradation information, and a second thermal heater that selectivelyheats a localized area, on which the second predetermined pressure isexerted by the second pressure applicator, to the second predeterminedtemperature in accordance with the first-single-color image-pixelinformation carrying the gradation information.

In the second aspect of the present invention, the layer ofmicrocapsules also may comprise a third type of microcapsule filled witha first type of second-single-color dye, and a fourth type ofmicrocapsule filled with a second type of second-single-color dye. Thethird type of microcapsule exhibits a third temperature/pressurecharacteristic such that, when the third type of microcapsule issquashed under a third predetermined pressure at a third predeterminedtemperature, discharge of the first type of second-single-color dye fromthe squashed microcapsule occurs. The fourth type of microcapsuleexhibits a fourth temperature/pressure characteristic such that, whenthe fourth type of microcapsule is squashed under a fourth predeterminedpressure at a fourth predetermined temperature, discharge of the secondtype of second-single-color dye from the squashed microcapsule occurs.

According to the second aspect of the present invention, to form animage on the image-forming substrate featuring the first, second, thirdand fourth types of microcapsules, there is provided an image-formingapparatus which comprises a first pressure applicator that exerts thefirst predetermined pressure on the image-forming substrate, a secondpressure applicator that exerts the second predetermined pressure on theimage-forming substrate, a third pressure applicator that exerts thethird predetermined pressure on the image-forming substrate, a fourthpressure applicator that exerts the fourth predetermined pressure on theimage-forming substrate, a first thermal heater that selectively heats alocalized area, on which the first predetermined pressure is exerted bythe first pressure applicator, to the first predetermined temperature inaccordance with first-single-color image-pixel information carryinggradation information, a second thermal heater that selectively heats alocalized area, on which the second predetermined pressure is exerted bythe second pressure applicator, to the second predetermined temperaturein accordance with the first-single-color image-pixel informationcarrying the gradation information, a third thermal heater thatselectively heats a localized area, on which the third predeterminedpressure is exerted by the third pressure applicator, to the thirdpredetermined temperature in accordance with second-single-colorimage-pixel information carrying gradation information, and a fourththermal heater that selectively heats a localized area, on which thefourth predetermined pressure is exerted by the fourth pressureapplicator, to the fourth predetermined temperature in accordance withthe second-single-color image-pixel information carrying the gradationinformation.

In the second aspect of the present invention, the layer ofmicrocapsules may further comprise a fifth type of microcapsule filledwith a first type of third-single-color dye, and a sixth type ofmicrocapsule filled with a second type of third-single-color dye. Thefifth type of microcapsule exhibits a fifth temperature/pressurecharacteristic such that, when the fifth type of microcapsule issquashed under a fifth predetermined pressure at a fifth predeterminedtemperature, discharge of the first type of third-single-color dye fromthe squashed microcapsule occurs. The sixth type of microcapsuleexhibits a sixth temperature/pressure characteristic such that, when thesixth type of microcapsule is squashed under a sixth predeterminedpressure at a sixth predetermined temperature, discharge of the secondtype of third-single-color dye from the squashed microcapsule occurs.

According to the second aspect of the present invention, to form animage on the image-forming substrate featuring the first, second, third,fourth, fifth and sixth types of microcapsules, there is provided animage-forming apparatus which comprises a first pressure applicator thatexerts the first predetermined pressure on the image-forming substrate,a second pressure applicator that exerts the second predeterminedpressure on the image-forming substrate, a third pressure applicatorthat exerts the third predetermined pressure on the image-formingsubstrate, a fourth pressure applicator that exerts the fourthpredetermined pressure on the image-forming substrate, a fifth pressureapplicator that exerts the fifth predetermined pressure on theimage-forming substrate, a sixth pressure applicator that exerts thesixth predetermined pressure on the image-forming substrate, a firstthermal heater that selectively heats a localized area, on which thefirst predetermined pressure is exerted by the first pressureapplicator, to the first predetermined temperature in accordance withfirst-single-color image-pixel information carrying gradationinformation, a second thermal heater that selectively heats a localizedarea, on which the second predetermined pressure is exerted by thesecond pressure applicator, to the second predetermined temperature inaccordance with the first-single-color image-pixel information carryingthe gradation information, a third thermal heater that selectively heatsa localized area, on which the third predetermined pressure is exertedby the third pressure applicator, to the third predetermined temperaturein accordance with second-single-color image-pixel information carryinggradation information, a fourth thermal heater that selectively heats alocalized area, on which the fourth predetermined pressure is exerted bythe fourth pressure applicator, to the fourth predetermined temperaturein accordance with the second-single-color image-pixel informationcarrying the gradation information, a fifth thermal heater thatselectively heats a localized area, on which the fifth predeterminedpressure is exerted by the fifth pressure applicator, to the fifthpredetermined temperature in accordance with third-single-colorimage-pixel information carrying gradation information, and a sixththermal heater that selectively heats a localized area, on which thesixth predetermined pressure is exerted by the sixth pressureapplicator, to the sixth predetermined temperature in accordance withthe third-single-color image-pixel information carrying the gradationinformation.

Similar to the first aspect of the present invention, if necessary, thefirst-single-color dye, the second-single-color dye and thethird-single-color dye comprise a same single-color dye exhibitingdiffering densities. Preferably, the first-single-color dye, thesecond-single-color dye and the third-single-color dye comprisesthree-primary color dyes. In this case, the image-forming substrate mayfurther comprise an additional layer of microcapsules filled with blackdye coated over the layer of microcapsules, and the microcapsules,included in the additional layer of microcapsules, are formed of resinsuch that they are at least thermally plasticized at a greatertemperature than the sixth predetermined temperature and under a lowerpressure than the sixth predetermined pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

These object and other objects of the present invention will be betterunderstood from the following description, with reference to theaccompanying drawings in which:

FIG. 1 is a schematic conceptual cross-sectional view showing a firstembodiment of an image-forming substrate, according to the presentinvention, coated with a layer of microcapsules comprising six types ofmicrocapsules filled with dyes exhibiting differing densities;

FIG. 2 is a schematic cross-sectional view showing different shell wallthicknesses of the six-types of microcapsules included in the layer ofmicrocapsules shown in FIG. 1;

FIG. 3 is a graph showing a characteristic curve of a longitudinalelasticity coefficient of a shape memory resin;

FIG. 4 is a graph showing temperature/pressure compactingcharacteristics of the six types of microcapsules included in the layerof microcapsules shown in FIG. 1;

FIG. 5 a schematic cross-sectional view of a color printer for forming acolor image on the image-forming substrate shown in FIG. 1;

FIG. 6 is a partial schematic block diagram of a control circuit for thecolor printer of FIG. 5, which shows representatively a driver circuitused in three line thermal heads of the color printer;

FIG. 7 is a timing chart showing a strobe signal and a control signalfor electronically actuating the thermal head driver circuit forproducing a yellow dot on the image-forming substrate of FIG. 1;

FIG. 8 is a table showing a relationship between a digital yellowimage-pixel signal carrying a 2-bit gradation-signal for producing thecontrol signal and a variation of the control signal caused by acombination of the digital yellow image-pixel signal and the 2-bitgradation-signal;

FIG. 9 is a conceptual view showing an example of variation in density(gradation) of a yellow dot produced on the image-forming substrate ofFIG. 1;

FIG. 10 is a conceptual view showing another example of variation indensity (gradation) of a yellow dot produced on the image-formingsubstrate of FIG. 1;

FIG. 11 is a timing chart showing a strobe signal and a control signalfor electronically actuating the thermal head driver circuit forproducing a magenta dot on the image-forming substrate of FIG. 1;

FIG. 12 is a table showing a relationship between a digital magentaimage-pixel signal carrying a 2-bit gradation-signal for producing thecontrol signal and a variation of the control signal caused by acombination of the digital magenta image-pixel signal and the 2-bitgradation-signal;

FIG. 13 is a timing chart showing a strobe signal and a control signalfor electronically actuating the thermal head driver circuit forproducing a cyan dot on the image-forming substrate of FIG. 1;

FIG. 14 is a table showing a relationship between a digital cyanimage-pixel signal carrying a 2-bit gradation-signal for producing thecontrol signal and a variation of the control signal caused by acombination of the digital cyan image-pixel signal and the 2-bitgradation-signal;

FIG. 15 is a schematic conceptual cross-sectional view showing a secondembodiment of an image-forming substrate, according to the presentinvention, coated with a layer of microcapsules comprising six types ofmicrocapsules filled with dyes exhibiting differing densities;

FIG. 16 is a schematic cross-sectional view showing different shell wallthicknesses of the six types of microcapsules included in the layer ofmicrocapsules shown in FIG. 15;

FIG. 17 is a graph showing temperature/pressure compactingcharacteristics of the six types of microcapsules included in the layerof microcapsules shown in FIG. 15;

FIG. 18 a schematic cross-sectional view of a color printer for forminga color image on the image-forming substrate shown in FIG. 15;

FIG. 19 is a partial schematic block diagram of a control circuit forthe color printer of FIG. 18, which shows representatively a set ofdriver circuits used in three sets of line thermal heads of the colorprinter;

FIG. 20 is a timing chart showing a set of strobe signals and a set ofcontrol signals for electronically actuating the set of thermal headdriver circuits for producing a yellow dot on the image-formingsubstrate of FIG. 15;

FIG. 21 is a table showing a relationship between a digital yellowimage-pixel signal carrying a 2-bit gradation-signal for producing theset of control signals and a variation of the sets of control signalscaused by a combination of the digital yellow image-pixel signal and the2-bit gradation-signal;

FIG. 22 is a conceptual view showing an example of variation in density(gradation) of a yellow dot produced on the image-forming substrate ofFIG. 15;

FIG. 23 is a conceptual view showing another example of variation indensity (gradation) of a yellow dot produced on the image-formingsubstrate of FIG. 15;

FIG. 24 is a conceptual view showing yet another example of variation indensity (gradation) of a yellow dot produced on the image-formingsubstrate of FIG. 15;

FIG. 25 is a timing chart showing a set of strobe signals and a set ofcontrol signals for electronically actuating the set of thermal headdriver circuits for producing a magenta dot on the image-formingsubstrate of FIG. 15;

FIG. 26 is a table showing a relationship between a digital magentaimage-pixel signal carrying a 2-bit gradation-signal for producing theset of control signals and a variation of the sets of control signalscaused by a combination of the digital magenta image-pixel signal andthe 2-bit gradation-signal;

FIG. 27 is a timing chart showing a set of strobe signals and a set ofcontrol signals for electronically actuating the set of thermal headdriver circuits for producing a cyan dot on the image-forming substrateof FIG. 15;

FIG. 28 is a table showing a relationship between a digital cyanimage-pixel signal carrying a 2-bit gradation-signal for producing theset of control signals and a variation of the sets of control signalscaused by a combination of the digital cyan image-pixel signal and the2-bit gradation-signal; and

FIG. 29 is a schematic conceptual cross-sectional view showing amodification of the first and second embodiments of the image-formingsubstrates shown in FIGS. 1 and 15.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of an image-forming substrate, generallyindicated by reference 10, which is constituted in accordance with thepresent invention. The image-forming substrate 10 is produced in a formof paper sheet. In particular, the image-forming substrate or sheet 10comprises a sheet of paper 12, a layer of microcapsules 14 coated over asurface of the paper sheet 12, and a sheet of protective transparentfilm 16 covering the microcapsule layer 14. The microcapsule layer 14 isformed of a plurality of microcapsules 18 comprising six types ofmicrocapsules uniformly distributed over the surface of the paper sheet12.

FIG. 2 representatively shows the six types of microcapsules, indicatedby references 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂, respectively. Afirst type of microcapsules 18Y₁ is filled with a first type of yellowliquid dye Y₁; a second type of microcapsules 18Y₂ is filled with asecond type of yellow liquid dye Y₂; a third type of microcapsules 18M₁is filled with a first type of magenta liquid dye M₁; a fourth type ofmicrocapsules 18M₂ is filled with a second type of magenta liquid dyeM₂; a fifth type of microcapsules 18C₁ is filled with a first type ofcyan liquid dye C₁; and a sixth type of microcapsules 18C₂ is filledwith a second type of cyan liquid dye C₂. The first and second types ofyellow liquid dyes Y₁ and Y₂ may exhibit the same density or may exhibitdifferent densities; the first and second types of magenta liquid dyesM₁ and M₂ may exhibit the same density or may exhibit differentdensities; and the first and second types of cyan liquid dyes C₁ and C₂may exhibit the same density or may exhibit different densities.

In each type of microcapsule (18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁, 18C₂), ashell wall of a microcapsule is formed of a synthetic resin material,usually colored white, which is the same color as the paper sheet 14.Accordingly, if the paper sheet 14 is colored with a single colorpigment, the resin material of the microcapsules 18Y₁, 18Y₂, 18M₁, 18M₂,18C₁ and 18C₂ may be colored by the same single color pigment.

In order to produce each of the types of microcapsules 18Y₁, 18Y₂, 18M₁,18M₂, 18C₁ and 18C₂, a polymerization method, such as interfacialpolymerization, in-situ polymerization or the like, may be utilized. Ineither case, the microcapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂ mayhave an average diameter of several microns, for example, 5 μm to 10 μm.

For the uniform formation of the microcapsule layer 14, for example, thesame amounts of microcapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂ arehomogeneously mixed with a suitable binder solution to form asuspension, and the paper sheet 12 is coated with the binder solution,containing the suspension of microcapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁and 18C₂, by using an atomizer. In FIG. 1, for the convenience ofillustration, although the microcapsule layer 14 is shown as having athickness corresponding to the diameter of the microcapsules 18Y₁, 18Y₂,18M₁, 18M₂, 18C₁ and 18C₂, in reality, the six types of microcapsules18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂ overlay each other, and thus themicrocapsule layer 14 has a larger thickness than the diameter of asingle microcapsule 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ or 18C₂.

In the image-forming substrate or sheet 10 shown in FIG. 1, for theresin material of each type of microcapsule (18Y₁, 18Y₂, 18M₁, 18M₂,18C₁, 18C₂), a shape memory resin is utilized. For example, the shapememory resin is represented by a polyurethane-based-resin, such aspolynorbornene, trans-1,4-polyisoprene polyurethane. As other types ofshape memory resin, a polyimide-based resin, a polyamide-based resin, apolyvinyl-chloride-based resin, a polyester-based resin and so on arealso known.

In general, as shown in a graph of FIG. 3, the shape memory resinexhibits a coefficient of longitudinal elasticity, which abruptlychanges at a glass-transition temperature boundary T_(g). In the shapememory resin, Brownian movement of the molecular chains is stopped in alow-temperature area “a”, which is below the glass-transitiontemperature T_(g), and thus the shape memory resin exhibits a glass-likephase. On the other hand, Brownian movement of the molecular chainsbecomes increasingly energetic in a high-temperature area “b”, which isabove the glass-transition temperature T_(g), and thus the shape memoryresin exhibits a rubber elasticity.

The shape memory resin is named due to the following shape memorycharacteristic: once a mass of the shape memory resin is worked into afinished article in the low-temperature area “a”, and is heated tobeyond the glass-transition temperature T_(g), the article becomesfreely deformable. After the shaped article is deformed into anothershape, and cooled to below the glass-transition temperature T_(g), themost recent shape of the article is fixed and maintained. Nevertheless,when the deformed article is again heated to above the glass-transitiontemperature T_(g), without being subjected to any load or externalforce, the deformed article returns to the original shape.

In the image-forming substrate or sheet 10, the shape memorycharacteristic per se is not utilized, but the characteristic abruptchange of the shape memory resin in the longitudinal elasticitycoefficient is utilized, such that the six types of microcapsules 18Y₁,18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂ can be selectively squashed and brokenunder varying combinations of selected temperatures and pressures.

In particular, as shown in a graph of FIG. 4, a shape memory resin ofthe first type of microcapsules 18Y₁ is prepared so as to exhibit acharacteristic longitudinal elasticity coefficient, indicated by a solidline YL₁, having a glass-transition temperature T₁; a shape memory resinof the second type of microcapsules 18Y₂ is prepared so as to exhibit acharacteristic longitudinal elasticity coefficient, indicated by a solidline YL₂, having a glass-transition temperature T₂; a shape memory resinof the third type of microcapsules 18M₁ is prepared so as to exhibit acharacteristic longitudinal elasticity coefficient, indicated by adouble-chained line ML₁, having a glass-transition temperature T₃; ashape memory resin of the fourth type of microcapsules 18M₂ is preparedso as to exhibit a characteristic longitudinal elasticity coefficient,indicated by a double-chained line ML₂, having a gall-transitiontemperature T₄; a shape memory resin of the fifth type of microcapsules18C₁ is prepared so as to exhibit a characteristic longitudinalelasticity coefficient, indicated by a single-chained line CL₁, having aglass-transition temperature T₅; and a shape memory resin of the sixthtype of microcapsules 18C₂ is prepared so as to exhibit a characteristiclongitudinal elasticity coefficient, indicated by a single-chained lineCL₂, having a glass-transition temperature T₆.

Note, by suitably varying compositions of the shape memory resin and/orby selecting a suitable one from among various types of shape memoryresin, it is possible to obtain the respective shape memory resins, withthe glass-transition temperatures T₁, T₂, T₃, T₄, T₅ and T₆. By way ofnon-limiting example, the glass-transition temperature T₁ is preferablyselected from a temperature range of 65°-70° C., and the remainingtemperatures T₂-T₆ are set at increments of 20° C. higher. Thus, if T₁is at 68° C., then T₂, T₃, T₄, T₅ and T₆ are set to 88° C., 108° C.,128° C., 148° C., and 168° C., respectively.

As shown in FIG. 2, the microcapsule walls of the six types ofmicrocapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂, respectively, havediffering thicknesses W_(Y1), W_(Y2), W_(M1), W_(M2), W_(C1) and W_(C2).Namely, the thicknesses W_(Y1) and W_(Y2) of the first and second typesof microcapsules 18Y₁ and 18Y₂ are larger than the thicknesses W_(M1)and W_(M2) of the third and fourth types of microcapsules 18M₁ and 18M₂,and the thicknesses W_(M1) and W_(M2) of the third and fourth types ofmicrocapsules 18M₁ and 18M₂ are larger than the thicknesses W_(C1) andW_(C2) of the fifth and sixth types of microcapsules 18C₁ and 18C₂.

The thickness W_(Y1) of the first type of microcapsules 18Y₁ is selectedsuch that each microcapsule 18Y₁ is compacted and broken under abreaking pressure that lies between a critical breaking pressure P₃ andan upper limit pressure P_(UL) (FIG. 4), when each microcapsule 18Y₁ isheated to a temperature between the glass-transition temperatures T₁ andT₃. The thickness W_(Y2) of the second type of microcapsules 18Y₂ isselected such that each microcapsule 18Y₂ is compacted and broken undera breaking pressure that lies between the critical breaking pressure P₃and the upper limit pressure P_(UL) (FIG. 4), when each microcapsule18Y₂ is heated to a temperature between the glass-transitiontemperatures T₂ and T₃.

The thickness W_(M1) of the third type of microcapsules 18M₁ is selectedsuch that each microcapsule 18M₁ is compacted and broken under abreaking pressure that lies between critical breaking pressures P₂ andP₃ (FIG. 4), when each microcapsule 18M₁ is heated to a temperaturebetween the glass-transition temperatures T₃ and T₅. The thicknessW_(M2) of the fourth type of microcapsules 18M₂ is selected such thateach microcapsule 18M₂ is compacted and broken under a breaking pressurethat lies between the critical breaking pressures P₂ and P₃ (FIG. 4),when each microcapsule 18M₂ is heated to a temperature between theglass-transition temperatures T₄ and T₅.

The thickness W_(C1) of the fifth type of microcapsules 18C₁ is selectedsuch that each microcapsule 18C₁ is compacted and broken under abreaking pressure that lies between critical breaking pressures P₁ andP₂ (FIG. 4), when each microcapsule 18C₁ is heated to a temperaturebetween the glass-transition temperature T₅ and an upper limittemperature T_(UL) (FIG. 4). The thickness W_(C2) of the sixth type ofmicrocapsules 18C₂ is selected such that each microcapsule 18C₂ iscompacted and broken under a breaking pressure that lies between thecritical breaking pressures P₁ and P₂ (FIG. 4), when each microcapsule18C₂ is heated to a temperature between the glass-transition temperatureT₆ and the upper limit temperature T_(UL).

It is noted that when the glass transition temperatures T₁, T₂, T₃, T₄,T₅ and T₆ are set as mentioned above, the upper limit temperature T_(UL)is preferably selected from 185° C.-190° C., i.e., 20° C. above theselected T₆ temperature.

By way of non-limiting example, the pressures P₁, P₂, P₃, and P_(UL),are preferably set to 0.02, 0.2, 2.0, and 20 MPa, respectively.

As is apparent from the foregoing, by suitably selecting a heatingtemperature and a breaking pressure, which should be exerted on theimage-forming sheet 10, it is possible to selectively squash and breakthe first, second, third, fourth, fifth and sixth types of microcapsules18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂.

For example, if the selected heating temperature and breaking pressurefall within a hatched yellow area YA₁ (FIG. 4), defined by a temperaturerange between the glass-transition temperatures T₁ and T₂ and by apressure range between the critical breaking pressure P₃ and the upperlimit pressure P_(UL), only the first type of microcapsules 18Y₁ issquashed and broken. If the selected heating temperature and breakingpressure fall within a hatched yellow area YA₁/YA₂, defined by atemperature range between the glass-transition temperatures T₂ and T₃and by a pressure range between the critical breaking pressure P₃ andthe upper limit pressure P_(UL), both the first and second types ofmicrocapsules 18Y₁ and 18Y₂ are squashed and broken.

Also, if the selected heating temperature and breaking pressure fallwithin a hatched magenta area MA₁ (FIG. 4), defined by a temperaturerange between the glass-transition temperatures T₃ and T₄ and by apressure range between the critical breaking pressures P₂ and P₃, onlythe third type of microcapsules 18M₁ is squashed and broken. If theselected heating temperature and breaking pressure fall within a hatchedmagenta area MA₁/MA₂, defined by a temperature range between theglass-transition temperatures T₄ and T₅ and by a pressure range betweenthe critical breaking pressures P₂ and P₃, both the third and fourthtypes of microcapsules 18M₁ and 18M₂ are squashed and broken.

Further, if the selected heating temperature and breaking pressure fallwithin a hatched cyan area CA₁ (FIG. 4), defined by a temperature rangebetween the glass-transition temperatures T₅ and T₆ and by a pressurerange between the critical breaking pressures P₁ and P₂, only the fifthtype of microcapsules 18C₁ is squashed and broken. If the selectedheating temperature and breaking pressure fall within a hatched cyanarea CA₁/CA₂, defined by a temperature range between theglass-transition temperature T₆ and the upper limit temperature T_(UL)and by a pressure range between the critical breaking pressures P₁ andP₂, both the fifth and sixth types of microcapsules 18C₁ and 18C₂ aresquashed and broken.

FIG. 5 schematically shows a thermal color printer, which is constitutedas a line printer so as to form a color image on the image-forming sheet10, which features by the first, second, third, fourth, fifth and sixthtypes of microcapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂.

The color printer comprises a generally-rectangular parallelopipedhousing 20 having an entrance opening 22 and an exit opening 24 formedin a top wall and a side wall of the housing 20, respectively. Theimage-forming sheet 10 (not shown in FIG. 5) is introduced into thehousing 20 through the entrance opening 22, and is then discharged fromthe exit opening 24 after the formation of a color image on theimage-forming sheet 10. Note, in FIG. 5, a path 26 for movement of theimage-forming sheet 10 is indicated by a chained line.

A guide plate 28 is provided in the housing 20 so as to define a part ofthe path 26 for the movement of the image-forming sheet 10, and a firstthermal head 30Y, a second thermal head 30M and a third thermal head 30Care securely attached to a surface of the guide plate 28. The thermalheads 30Y, 30M and 30C are essentially identical to each other, and eachthermal head (30Y, 30M, 30C) is formed as a line thermal head extendingperpendicularly with respect to a direction of movement of theimage-forming sheet 10. Each of the thermal heads 30Y, 30M and 30Cincludes a plurality of heater elements or electric resistance elements,and these electric resistance elements are aligned with each other alonga length of the corresponding line thermal head (30Y, 30M, 30C).

The first thermal head 30Y is used to form a yellow-dotted image on theimage-forming sheet 10, and each of the electric resistance elementsthereof is selectively and electrically energized to produce ayellow-image-pixel dot in accordance with a digital yellow image-pixelsignal carrying a 2-bit digital gradation signal. When the digitalyellow image-pixel signal has a value “0”, the corresponding electricresistance element is not electrically energized. When the digitalyellow image-pixel signal has a value “1”, the corresponding electricresistance element is electrically energized and heated to either atemperature between the glass-transition temperatures T₁ and T₂ or atemperature between the glass-transition temperatures T₂ and T₃, inaccordance with the 2-bit digital gradation signal carried by thedigital yellow image-pixel signal.

The second thermal head 30M is used to form a magenta-dotted image onthe image-forming sheet 10, and each of the electric resistance elementsthereof is selectively and electrically energized to produce amagenta-image-pixel dot in accordance with a digital magenta image-pixelsignal carrying a 2-bit digital gradation signal. When the digitalmagenta image-pixel signal has a value “0”, the corresponding electricresistance element is not electrically energized. When the digitalmagenta image-pixel signal has a value “1”, the corresponding electricresistance element is electrically energized and heated to either atemperature between the glass-transition temperatures T₃ and T₄ or atemperature between the glass-transition temperatures T₄ and T₅, inaccordance with the 2-bit digital gradation signal carried by thedigital magenta-pixel signal.

The third thermal head 30C is used to form a cyan-dotted image on theimage-forming sheet 10, and each of the electric resistance elementsthereof is selectively and electrically energized to produce acyan-image-pixel dot in accordance with a digital cyan image-pixelsignal carrying a 2-bit digital gradation signal. When the digital cyanimage-pixel signal has a value “0”, the corresponding electricresistance element is not electrically energized. When the digital cyanimage-pixel signal has a value “1”, the corresponding electricresistance element is electrically energized and heated to either atemperature between the glass-transition temperatures T₅ and T₆ or atemperature between the glass-transition temperatures T₆ and the upperlimit temperature T_(UL), in accordance with the 2-bit digital gradationsignal carried by the digital cyan image-pixel signal.

Note, the line thermal heads 30Y, 30M and 30C are arranged in sequenceso that the respective heating temperatures increase in the movementdirection of the image-forming sheet 10.

The color printer further comprises a first roller platen 32Y, a secondroller platen 32M and a third roller platen 32C associated with thefirst, second and third thermal heads 30Y, 30M and 30C, respectively,and each of the roller platens 32Y, 32M and 32C may be formed of asuitable hard rubber material. The first roller platen 32Y is providedwith a first spring-biasing unit 34Y so as to be elastically pressedagainst the first thermal head 30Y at a pressure between the criticalcompacting-pressure P₃ and the upper limit pressure P_(UL); the secondroller platen 32M is provided with a second spring-biasing unit 34M soas to be elastically pressed against the second thermal head 30M at apressure between the critical compacting-pressures P₂ and P₃; and thethird roller platen 32C is provided with a third spring-biasing unit 34Cso as to be elastically pressed against the third thermal head 30C at apressure between the critical compacting-pressures P₁ and P₂.

Note, the platens 32Y, 32M and 32C are arranged in sequence so that therespective pressures, exerted by the platens 32Y, 32M and 32C on theline thermal heads 30Y, 30M and 30C, decrease in the movement directionof the image-forming sheet 10.

In FIG. 5, reference 36 indicates a control circuit board forcontrolling a printing operation of the color printer, and reference 38indicates a main electrical power source for electrically energizing thecontrol circuit board 36.

FIG. 6 shows a part of a schematic block diagram of the control circuitboard 36. As shown in this drawing, the control circuit board 36comprises a printer controller 40 including a microcomputer. The printercontroller 40 receives a series of digital color image-pixel signalsfrom a personal computer or a word processor (not shown) through aninterface circuit (I/F) 42, with each of the digital color image-pixelsignals carrying a digital 2-bit gradation-signal. The received digitalcolor image-pixel signals (i.e., digital cyan image-pixel signalscarrying 2-bit digital gradation signals, digital magenta image-pixelsignals carrying 2-bit digital gradation signals, and digital yellowimage-pixel signals carrying 2-bit digital gradation signals) are oncestored in a memory 44.

Also, the control circuit board 36 is provided with a motor drivercircuit 46 for driving three electric motors 48Y, 48M and 48C, which areused to rotationally drive the roller platens 32Y, 32M and 32C,respectively. In this embodiment of the color printer, each of themotors 48Y, 48M and 48C is a stepping motor, which is driven inaccordance with a series of drive pulses outputted from the motor drivercircuit 46, the outputting of drive pulses from the motor driver circuit46 to the motors 48Y, 48M and 48C being controlled by the printercontroller 40.

During a printing operation, the respective roller platens 32Y, 32M and32C are rotated in a counterclockwise direction (FIG. 5) by the motors48Y, 48M and 48C, with a same peripheral speed. Accordingly, theimage-forming sheet 10, introduced through the entrance opening 22,moves toward the exit opening 24 along the path 26.

Thus, the image-forming sheet 10 is subjected to pressure rangingbetween the critical compacting-pressure P₃ and the upper limit pressureP_(UL) when passing between the first thermal head 30Y and the firstroller platen 32Y; to pressure ranging between the criticalcompacting-pressures P₂ and P₃ when passing between the second thermalhead 30M and the second roller platen 32M; and to pressure rangingbetween the critical compacting-pressures P₁ and P₂ when passing betweenthe third thermal head 30C and the third roller platen 32C.

Note, in this embodiment of the color printer, the introduction of theimage-forming sheet 10 into the entrance opening 22 of the printer iscarried out such that the transparent protective film sheet 16 of theimage-forming sheet 10 comes into contact with the thermal heads 30Y,30M and 30C.

In FIG. 6, only one of the electric resistance elements, included in theline thermal heads 30Y, 30M and 30C, is representatively illustrated,and is indicated by reference ER. The electric resistance element ER isselectively and electrically energized by a driver circuit 50 undercontrol of the printer controller 40. The driver circuit 50 includes anAND-gate circuit 52 and a transistor 54. As shown in FIG. 6, a set of astrobe signal (STC, STM or STY) and a control signal (DAC, DAM or DAY)is inputted from the printer controller 40 to two input terminals of theAND-gate circuit 52. A base of the transistor 54 is connected to anoutput terminal of the AND-gate circuit 52; a collector of thetransistor 54 is connected to an electric power source (V_(cc)); and anemitter of the transistor 54 is connected to the electric resistanceelement ER.

When the electric resistance element ER, as shown in FIG. 6, is oneincluded in the first thermal head 30Y, a set of a strobe signal “STY”and a control signal “DAY” is outputted from the printer controller 40,and is then inputted to the input terminals of the AND-gate circuit 52,during a printing operation. As shown in a timing chart of FIG. 7, thestrobe signal “STY” has a pulse width “PWY”, and the control signal“DAY” is varied in accordance with binary values of a digital yellowimage-pixel signal “Y” and a 2-bit digital gradation signal “GSY”carried thereby, as shown in a table of FIG. 8.

Namely, when the digital yellow image-pixel signal “Y” has a value “0”,and when the 2-bit digital gradation signal “GSY” has a value [00], thecontrol signal “DAY” is maintained at a low-level under control of theprinter controller 40 (FIGS. 7 and 8). When the digital yellowimage-pixel signal has a value “1”, the control signal “DAY” isoutputted as a high-level pulse from the printer controller 40, and apulse width of the high-level pulse is varied in accordance with a valueof the 2-bit digital gradation signal “GSY”.

In particular, when the 2-bit digital gradation signal “GSY” has a valueof [01], the high-level pulse of the control signal “DAY” has a pulsewidth “PWY₁” shorter than the pulse width “PWY” of the strobe signal“STY”. Thus, the electric resistance element ER is electricallyenergized during a period corresponding to the pulse width “PWY₁” of thehigh-level pulse of the control signal “DAY”, whereby the electricresistance element ER is heated to the temperature between theglass-transition temperatures T₁ and T₂. Accordingly, in a dot area,defined by the electric resistance element ER, on the image-formingsheet 10, only the microcapsules 18Y₁ are compacted and broken,resulting in seepage of the first type of yellow liquid dye Y₁ from thecompacted and broken microcapsules 18Y₁ in the dot area, as shown inFIG. 9. Namely, a yellow dot, which is colored by only the first type ofyellow liquid dye Y₁, is produced on the image-forming sheet 10.

Also, when the 2-bit digital gradation signal “GSY” has a value of [10],the high-level pulse of the control signal “DAY” has the same pulsewidth “PWY₂” as the pulse width “PWY” of the strobe signal “STY”. Thus,the electric resistance element ER is electrically energized during aperiod corresponding to the pulse width “PWY₂” of the high-level pulseof the control signal “DAY”, whereby the electric resistance element ERis heated to the temperature between the glass-transition temperaturesT₂ and T₃. Accordingly, in a dot area, defined by the electricresistance element ER, on the image-forming sheet 10, both themicrocapsules 18Y₁ and the microcapsules 18Y₂ are compacted and broken,resulting in seepage of the first and second types of yellow liquid dyesY₁ and Y₂ from the compacted and broken microcapsules 18Y₁ and 18Y₂, asshown in FIG. 10. Namely, a yellow dot, which is colored by both thefirst and second types of yellow liquid dyes Y₁ and Y₂, is produced onthe image-forming sheet 10.

Of course, a yellow density of the yellow dot, colored by only the firsttype of yellow dye Y₁, is different from that of the yellow dot coloredby both the first and second types of yellow liquid dyes Y₁ and Y₂,thereby obtaining a variation in density (gradation) of the yellow dot.

When the electric resistance element ER, as shown in FIG. 6, is oneincluded in the second thermal head 30M, a set of a strobe signal “STM”and a control signal “DAM” is outputted from the printer controller 40,and is then inputted to the input terminals of the AND-gate circuit 52,during a printing operation. As shown in a timing chart of FIG. 11, thestrobe signal “STM” has a pulse width “PWM”, longer than the pulse widthof the strobe signal “STY” (FIG. 7), and the control signal “DAM” isvaried in accordance with binary values of a digital magenta image-pixelsignal “M” and a 2-bit digital gradation signal “GSM” carried thereby,as shown in a table of FIG. 12.

Namely, when the digital magenta image-pixel signal “M” has a value “0”,and when the 2-bit digital gradation signal “GSM” has a value [00], thecontrol signal “DAM” is maintained at a low-level under control of theprinter controller 40 (FIGS. 11 and 12). When the digital magentaimage-pixel signal has a value “1”, the control signal “DAM” isoutputted as a high-level pulse from the printer controller 40, and apulse width of the high-level pulse is varied in accordance with a valueof the 2-bit digital gradation signal “GSM”.

In particular, when the 2-bit digital gradation signal “GSM” has a valueof [01], the high-level pulse of the control signal “DAM” has a pulsewidth “PWM₁” shorter than the pulse width “PWM” of the strobe signal“STM”. Thus, the electric resistance element ER is electricallyenergized during a period corresponding to the pulse width “PWM₁” of thehigh-level pulse of the control signal “DAM”, whereby the electricresistance element ER is heated to the temperature between theglass-transition temperatures T₃ and T₄. Accordingly, in a dot area,defined by the electric resistance element ER, on the image-formingsheet 10, only the microcapsules 18M₁ are compacted and broken,resulting in seepage of the first type of magenta liquid dye M₁ from thecompacted and broken microcapsules 18M₁ in the dot area. Namely, amagenta dot, which is colored by only the first type of magenta liquiddye M₁, is produced on the image-forming sheet 10.

Also, when the 2-bit digital gradation signal “GSM” has a value of [10],the high-level pulse of the control signal “DAM” has the same pulsewidth “PWM₂”as the pulse width “PWM” of the strobe signal “STM”. Thus,the electric resistance element ER is electrically energized during aperiod corresponding to the pulse width “PWM₂” of the high-level pulseof the control signal “DAM”, whereby the electric resistance element ERis heated to the temperature between the glass-transition temperaturesT₄ and T₅. Accordingly, in a dot area, defined by the electricresistance element ER, on the image-forming sheet 10, both themicrocapsules 18M₁ and the microcapsules 18M₂ are compacted and broken,resulting in seepage of the first and second types of magenta liquiddyes M₁ and M₂ from the compacted and broken microcapsules 18M₁ and18M₂. Namely, a magenta dot, which is colored by both the first andsecond types of magenta liquid dyes M₁ and M₂, is produced on theimage-forming sheet 10.

Of course, a magenta density of the magenta dot, colored by only thefirst type of magenta dye M₁, is different from that of the magenta dotcolored by both the first and second types of magenta liquid dyes M₁ andM₂, thereby obtaining a variation in density (gradation) of the magentadot.

When the electric resistance element ER, as shown in FIG. 6, is oneincluded in the third thermal head 30C, a set of a strobe signal “STC”and a control signal “DAC” is outputted from the printer controller 40,and is then inputted to the input terminals of the AND-gate circuit 52,during a printing operation. As shown in a timing chart of FIG. 13, thestrobe signal “STC” has a pulse width “PWC”, longer than the pulse widthof the strobe signal “STM” (FIG. 11), and the control signal “DAC” isvaried in accordance with binary values of a digital cyan image-pixelsignal “C” and a 2-bit digital gradation signal “GSC” carried thereby,as shown in a table of FIG. 14.

Namely, when the digital cyan image-pixel signal “C” has a value “0”,and when the 2-bit digital gradation signal “GSC” has a value [00], thecontrol signal “DAC” is maintained at a low-level under control of theprinter controller 40 (FIGS. 13 and 14). When the digital cyanimage-pixel signal has a value “1”, the control signal “DAC” isoutputted as a high-level pulse from the printer controller 40, and apulse width of the high-level pulse is varied in accordance with a valueof the 2-bit digital gradation signal “GSC”.

In particular, when the 2-bit digital gradation signal “GSC” has a valueof [01], the high-level pulse of the control signal “DAC” has a pulsewidth “PWC₁” shorter than the pulse width “PWC” of the strobe signal“STC” . Thus, the electric resistance element ER is electricallyenergized during a period corresponding to the pulse width “PWC₁” of thehigh-level pulse of the control signal “DAC”, whereby the electricresistance element ER is heated to the temperature between theglass-transition temperatures T₅ and T₆. Accordingly, in a dot area,defined by the electric resistance element ER, on the image-formingsheet 10, only the microcapsules 18C₁ are compacted and broken,resulting in seepage of the first type of cyan liquid dye C₁ from thecompacted and broken microcapsules 18C₁ in the dot area. Namely, a cyandot, which is colored by only the first type of cyan liquid dye C₁, isproduced on the image-forming sheet 10.

Also, when the 2-bit digital gradation signal “GSC” has a value of [10],the high-level pulse of the control signal “DAC” has the same pulsewidth “PWC₂” as the pulse width “PWC” of the strobe signal “STC”. Thus,the electric resistance element ER is electrically energized during aperiod corresponding to the pulse width “PWC₂” of the high-level pulseof the control signal “DAC”, whereby the electric resistance element ERis heated to the temperature between the glass-transition temperaturesT₆ and the upper limit temperature T_(UL). Accordingly, in a dot area,defined by the electric resistance element ER, on the image-formingsheet 10, both the microcapsules 18C₁ and the microcapsules 18C₂ arecompacted and broken, resulting in seepage of the first and second typesof cyan liquid dyes C₁ and C₂ from the compacted and brokenmicrocapsules 18C₁ and 18C₂. Namely, a cyan dot, which is colored byboth the first and second types of cyan liquid dyes C₁ and C₂, isproduced on the image-forming sheet 10.

Of course, a cyan density of the cyan dot, colored by only the firsttype of cyan dye C₁, is different from that of the cyan dot colored byboth the first and second types of cyan liquid dyes C₁ and C₂, therebyobtaining a variation in density (gradation) of the cyan dot.

Note, the yellow dot, the magenta dot and the cyan dot, produced on theimage-forming sheet 10, have a dot size of about 50 μm to about 100 μm,and the first, second, third, fourth, fifth and sixth types ofmicrocapsules 18Y₁, 18Y₂, 18M₁, 18M₂, 18C₁ and 18C₂ are uniformlyincluded in a dot area to be produced on the image-forming sheet 10.

FIG. 15 shows a second embodiment of an image-forming substrate,generally indicated by reference 10′, which is constituted in accordancewith the present invention. The image-forming substrate 10′ is alsoproduced in a form of paper sheet. Namely, the image-forming substrateor sheet 10′ comprises a sheet of paper 12′, a layer of microcapsules14′ coated over a surface of the paper sheet 12′, and a sheet ofprotective transparent film 16′ covering the microcapsule layer 14′. Themicrocapsule layer 14′ is formed of a plurality of microcapsules 18′comprising six types of microcapsules uniformly distributed over thesurface of the paper sheet 12′.

FIG. 16 representatively shows the six types of microcapsules, indicatedby references 18Y₁′, 18Y₂′, 18M₁′, 18M₂′, 18C₁′ and 18C₂′. Similar tothe first embodiment, a first type of microcapsules 18Y₁′ is filled witha first type of yellow liquid dye Y₁; a second type of microcapsules18Y₂′ is filled with a second type of yellow liquid dye Y₂; a third typeof microcapsules 18M₁′ is filled with a first type of magenta liquid dyeM₁; a fourth type of microcapsules 18M₂′ is filled with a second type ofmagenta liquid dye M₂; a fifth type of microcapsules 18C₁′ is filledwith a first type of cyan liquid dye C₁; and a sixth type ofmicrocapsules 18C₂ is filled with a second type of cyan liquid dye C₂.

Note, in this second embodiment, the first and second yellow types ofliquid dyes Y₁ and Y₂ exhibit different densities; the first and secondtypes of magenta liquid dyes M₁ and M₂ exhibit different densities; andthe first and second types of cyan liquid dyes C₁ and C₂ exhibitdifferent densities.

The six types of microcapsules 18Y₁′, 18Y₂′, 18M₁′, 18M₂′, 18C₁′ and18C₂′ may be produced by the same polymerization method as mentionedpreviously, and may have an average diameter of several microns, forexample, 5 μm to 10 μm. Also, by using these six types of microcapsules(18Y₁′, 18Y₂′, 18M₁′, 18M₂′, 18C₁′ and 18C₂′), the uniform formation ofthe microcapsule layer 14′ is performed in the same manner as mentionedabove.

Similar to the first embodiment, for the resin material of each type ofmicrocapsule (18Y₁′, 18Y₂′, 18M₁′, 18M₂′, 18C₁′, 18C₂′), a shape memoryresin is utilized, but these shape memory resins exhibit characteristiclongitudinal elasticity coefficients different from those shown in FIG.4.

In particular, as shown in a graph of FIG. 17, a shape memory resin ofthe first type of microcapsules 18Y₁′ is prepared so as to exhibit acharacteristic longitudinal elasticity coefficient, indicated by a solidline YL₁′, having a glass-transition temperature TT₁; a shape memoryresin of the second type of microcapsules 18Y₂′ is prepared so as toexhibit a characteristic longitudinal elasticity coefficient, indicatedby a solid line YL₂′, having a glass-transition temperature TT₂; a shapememory resin of the third type of microcapsules 18M₁′ is prepared so asto exhibit a characteristic longitudinal elasticity coefficient,indicated by a double-chained line ML₁′, having a glass-transitiontemperature TT₃; a shape memory resin of the fourth type ofmicrocapsules 18M₂′ is prepared so as to exhibit a characteristiclongitudinal elasticity coefficient, indicated by a double-chained lineML₂′, having a glass-transition temperature TT₄; a shape memory resin ofthe fifth type of microcapsules 18C₁′ is prepared so as to exhibit acharacteristic longitudinal elasticity coefficient, indicated by asingle-chained line CL₁′, having a glass-transition temperature TT₅; anda shape memory resin of the sixth type of microcapsules 18C₂ is preparedso as to exhibit a characteristic longitudinal elasticity coefficient,indicated by a single-chained line CL₂′, having a glass-transitiontemperature TT₆.

As shown in FIG. 16, the microcapsule walls of the six types ofmicrocapsules 18Y₁′, 18Y₂′, 18M₁′, 18M₂′, 18C₁′ and 18C₂′, respectively,have differing thicknesses W_(Y1)′, W_(Y2)′, W_(M1)′, W_(M2)′, W_(C1)′and W_(C2)′. Namely, the thicknesses W_(Y1)′ and W_(Y2)′ of the firstand second types of microcapsules 18Y₁′ and 18Y₂′ are larger than thethicknesses W_(M1)′ and W_(M2)′ of the third and fourth types ofmicrocapsules 18M₁′ and 18M₂′, and the thicknesses W_(M1)′ and W_(M2)′of the third and fourth types of microcapsules 18M₁′ and 18M₂′ arelarger than the thicknesses W_(C1)′ and W_(C2)′ of the fifth and sixthtypes of microcapsules 18C₁′ and 18C₂′.

The thickness W_(Y1)′ of the first type of microcapsules 18Y₁′ isselected such that each microcapsule 18Y₁′ is compacted and broken undera breaking pressure that lies between a critical breaking pressure PP₆and an upper limit pressure PP_(UL) (FIG. 17), when each microcapsule18Y₁′ is heated to a temperature between the glass-transitiontemperatures TT₁ and TT₂. The thickness W_(Y2)′ of the second type ofmicrocapsules 18Y₂′ is selected such that each microcapsule 18Y₂′ iscompacted and broken under a breaking pressure that lies between acritical breaking pressure PP₅ and the critical breaking pressure PP₆(FIG. 17), when each microcapsule 18Y₂′ is heated to a temperaturebetween the glass-transition temperatures TT₂ and TT₃.

The thickness W_(M1)′ of the third type of microcapsules 18M₁′ isselected such that each microcapsule 18M₁′ is compacted and broken undera breaking pressure that lies between a critical breaking pressure PP₄and the critical breaking pressure PP₅ (FIG. 17), when each microcapsule18M₁′ is heated to a temperature between the glass-transitiontemperatures TT₃ and TT₄. The thickness W_(M2)′ of the fourth type ofmicrocapsules 18M₂′ is selected such that each microcapsule 18M₂′ iscompacted and broken under a breaking pressure that lies between acritical breaking pressure PP₃ and the critical breaking pressure PP₄(FIG. 17), when each microcapsule 18M₂′ is heated to a temperaturebetween the glass-transition temperatures TT₄ and TT₅.

The thickness W_(C1)′ of the fifth type of microcapsules 18C₁′ isselected such that each microcapsule 18C₁ ′ is compacted and brokenunder a breaking pressure that lies between a critical breaking pressurePP₂ and the critical breaking pressure PP₃ (FIG. 17), when eachmicrocapsule 18C₁′ is heated to a temperature between theglass-transition temperatures TT₅ and TT₆ (FIG. 17). The thicknessW_(C2)′ of the sixth type of microcapsules 18C₂ is selected such thateach microcapsule 18C₂′ is compacted and broken under a breakingpressure that lies between a critical breaking pressure PP₁ and thecritical breaking pressure PP₂ (FIG. 17), when each microcapsule 18C₂′is heated to a temperature between the glass-transition temperature TT₆and an upper limit temperature TT_(UL).

Similar to the first embodiment, the temeperature TT₁ is preferablyselected from the range of 65° C.-70° C., while the remainingtemperatures TT₂-TT_(UL) are set at incrementing 20° C. intervals. Thus,if TT₁, is 70° C., then TT₂, TT₃, TT₄, TT₅, TT₆ And TT_(UL) are set to90° C., 110° C., 130° C., 150° C., 170° C., and 190° C. respectively.Corresponding pressures PP₁, PP_(2, PP) ₃, PP₄, PP₅, PP₆ And PP_(UL)are, by way of non-limiting example, set to 0.02, 0.1, 0.2, 1.0, 2.0,10, and 20 MPas, respectively.

For example, if the selected heating temperature and breaking pressurefall within a hatched yellow area YA₁′ (FIG. 17), defined by atemperature range between the glass-transition temperatures TT₁ and TT₂and by a pressure range between the critical breaking pressure PP₆ andthe upper limit pressure PP_(UL), only the first type of microcapsules18Y₁′ is squashed and broken. If the selected heating temperature andbreaking pressure fall within a hatched yellow area YA₂′, defined by atemperature range between the glass-transition temperatures TT₂ and TT₃and by a pressure range between the critical breaking pressures PP₅ andPP₆, only the second type of microcapsules 18Y₂′ are squashed andbroken.

Also, if the selected heating temperature and breaking pressure fallwithin a hatched magenta area MA₁′ (FIG. 17), defined by a temperaturerange between the glass-transition temperatures TT₃ and TT₄ and by apressure range between the critical breaking pressures PP₄ and PP₅, onlythe third type of microcapsules 18M₁′ is squashed and broken. If theselected heating temperature and breaking pressure fall within a hatchedmagenta area MA₂′, defined by a temperature range between theglass-transition temperatures TT₄ and TT₅ and by a pressure rangebetween the critical breaking pressures PP₃ and PP₄, only the fourthtype of microcapsules 18M₂′ is squashed and broken.

Further, if the selected heating temperature and breaking pressure fallwithin a hatched cyan area CA₁′ (FIG. 17), defined by a temperaturerange between the glass-transition temperatures TT₅ and TT₆ and by apressure range between the critical breaking pressures PP₂ and PP₃, onlythe fifth type of microcapsules 18C₁′ is squashed and broken. If theselected heating temperature and breaking pressure fall within a hatchedcyan area CA₂′, defined by a temperature range between theglass-transition temperature TT₆ and the upper limit temperature TT_(UL)and by a pressure range between the critical breaking pressures PP₁ andPP₂, only the sixth type of microcapsules 18C₂′ are squashed and broken.

FIG. 18 schematically shows a thermal color printer, which isconstituted as a line printer so as to form a color image on theimage-forming sheet 10′, which features by the first, second, third,fourth, fifth and sixth types of microcapsules 18Y₁′, 18Y₂′, 18M₁′,18M₂′, 18C₁′ and 18C₂′. As is apparent from FIG. 18, this thermal lineprinter is similar to that shown in FIG. 5, and thus, in this drawing,the features similar to those of FIG. 5 are indicated by the samereference numerals.

The color printer also comprises a generally-rectangular parallelopipedhousing 20 having an entrance opening 22 and an exit opening 24 formedin a top wall and a side wall of the housing 20, respectively. Theimage-forming sheet 10′ (not shown in FIG. 18) is introduced into thehousing 20 through the entrance opening 22, and is then discharged fromthe exit opening 24 after the formation of a color image on theimage-forming sheet 10′. Note, in FIG. 18, a path 26 for movement of theimage-forming sheet 10′ is indicated by a chained line.

A guide plate 28 is provided in the housing 20 so as to define a part ofthe path 26 for the movement of the image-forming sheet 10′, and a firstset of thermal heads 30Y₁ and 30Y₂, a second set of thermal heads 30M₁and 30M₂ and a third thermal heads 30C₁ and 30C₂ are securely attachedto a surface of the guide plate 28. These thermal heads 30Y₁ and 30Y₂;30M₁ and 30M₂; and 30C₁ and 30C₂ are essentially identical to eachother, and each thermal head is formed as a line thermal head extendingperpendicularly with respect to a direction of movement of theimage-forming sheet 10′. Each of the thermal heads 30Y₁ and 30Y₂; 30M₁and 30M₂; and 30C₁ and 30C₂ includes a plurality of heater elements orelectric resistance elements, and these electric resistance elements arealigned with each other along a length of the corresponding line thermalhead (30Y₁, 30Y₂; 30M₁, 30M₂; 30C₁, 30C₂).

The first set of thermal heads 30Y₁ and 30Y₂ is used to form ayellow-dotted image on the image-forming sheet 10′, and a pair ofcorresponding electric resistance elements, included in the thermalheads 30Y₁ and 30Y₂, is selectively and electrically energized toproduce a yellow-image-pixel dot in accordance with a digital yellowimage-pixel signal carrying a 2-bit digital gradation signal. When thedigital yellow image-pixel signal has a value “0”, the correspondingpair of electric resistance elements are not electrically energized.When the digital yellow image-pixel signal has a value “1”, at least oneof the corresponding pair of electric resistance elements iselectrically energized in accordance with the 2-bit digital gradationsignal carried by the digital yellow image-pixel signal. In either case,whenever one of the electric resistance elements, included in thethermal head 30Y₁, is electrically energized, it is heated to atemperature between the glass-transition temperatures TT₁ and TT₂. Also,whenever one of the electric resistance elements, included in thethermal head 30Y₂ is electrically energized, it is heated to atemperature between the glass-transition temperatures TT₂ and TT₃.

The second set of thermal heads 30M₁ and 30M₂ is used to form amagenta-dotted image on the image-forming sheet 10′, and a pair ofcorresponding electric resistance elements, included in the thermalheads 30M₁ and 30M₂, is selectively and electrically energized toproduce a magenta-image-pixel dot in accordance with a digital magentaimage-pixel signal carrying a 2-bit digital gradation signal. When thedigital magenta image-pixel signal has a value “0”, the correspondingpair of electric resistance elements are not electrically energized.When the digital magenta image-pixel signal has a value “1”, at leastone of the corresponding pair of electric resistance elements iselectrically energized in accordance with the 2-bit digital gradationsignal carried by the digital magenta image-pixel signal. In eithercase, whenever one of the electric resistance elements, included in thethermal head 30M₁, is electrically energized, it is heated to atemperature between the glass-transition temperatures TT₃ and TT₄. Also,whenever one of the electric resistance elements, included in thethermal head 30M₂ is electrically energized, it is heated to atemperature between the glass-transition temperatures TT₄ and TT₅.

The third set of thermal heads 30C₁ and 30C₂ is used to form acyan-dotted image on the image-forming sheet 10′, and a pair ofcorresponding electric resistance elements, included in the thermalheads 30C₁ and 30C₂, is selectively and electrically energized toproduce a cyan-image-pixel dot in accordance with a digital cyanimage-pixel signal carrying a 2-bit digital gradation signal. When thedigital cyan image-pixel signal has a value “0”, the corresponding pairof electric resistance elements are not electrically energized. When thedigital cyan image-pixel signal has a value “1”, at least one of thecorresponding pair of electric resistance elements is electricallyenergized in accordance with the 2-bit digital gradation signal carriedby the digital cyan image-pixel signal. In either case, whenever one ofthe electric resistance elements, included in the thermal head 30C₁, iselectrically energized, it is heated to a temperature between theglass-transition temperatures TT₅ and TT₆. Also, whenever one of theelectric resistance elements, included in the thermal head 30C₂ iselectrically energized, it is heated to a temperature between theglass-transition temperature TT₆ and the upper limit temperatureTT_(UL).

Note, the line thermal heads 30Y₁ and 30Y₂; 30M₁ and 30M₂; and 30C₁ and30C₂ are arranged in sequence so that the respective heatingtemperatures increase in the movement direction of the image-formingsheet 10′.

The color printer further comprises a first set of roller platens 32Y₁and 32Y₂ associated with the first set of thermal heads 30Y₁ and 30Y₂, asecond set of roller platens 32M₁ and 32M₂ associated with the secondset thermal heads 30M₁ and 30M₂, and a third set of roller platens 32C₁and 32C₂ associated with the third set of thermal heads 30C₁ and 30C₂,and each of the roller platens 32Y₁ and 32Y₂; 32M₁ and 32M₂; and 32C₁and 30C₂ may be formed of a suitable hard rubber material.

The first set of roller platens 32Y₁ and 32Y₂ is provided with a firstset of spring-biasing units 34Y₁ and 34Y₂. The roller platen 32Y₁ iselastically pressed against the thermal head 30Y₁ by the spring-biasingunit 34Y₁ at a pressure between the critical compacting-pressure PP₆ andthe upper limit pressure PP_(UL), and the roller platen 32Y₂ iselastically pressed against the thermal head 30Y₂ by the spring-biasingunit 34Y₂ at a pressure between the critical compacting-pressures PP₅and PP₆.

The second set of roller platens 32M₁ and 32M₂ is provided with a secondset of spring-biasing units 34M₁ and 34M₂. The roller platen 32M₁ iselastically pressed against the thermal head 30M₁ by the spring-biasingunit 34M₁ at a pressure between the critical compacting-pressures PP₄and PP₅, and the roller platen 32M₂ is elastically pressed against thethermal head 30M₂ by the spring-biasing unit 34M₂ at a pressure betweenthe critical compacting-pressures PP₃ and PP₄.

The third set of roller platens 32C₁ and 32C₂ is provided with a thirdset of spring-biasing units 34C₁ and 34C₂. The roller platen 32C₁ iselastically pressed against the thermal head 30C₁ by the spring-biasingunit 34C₁ at a pressure between the critical compacting-pressures PP₂and PP₃, and the roller platen 32C₂ is elastically pressed against thethermal head 30C₂ by the spring-biasing unit 34C₂ at a pressure betweenthe critical compacting-pressures PP₁ and PP₂.

Note, the roller platens 32Y₁ and 32Y₂; 32M₁ and 32M₂; and 32C₁ and 32C₂are arranged in sequence so that the respective pressures, exerted bythe platens 32Y₁ and 32Y₂; 32M₁ and 32M₂; and 32C₁ and 32C₂ on the linethermal heads 30Y₁ and 30Y₂; 30M₁ and 30M₂; and 30C₁ and 30C₂, decreasein the movement direction of the image-forming sheet 10′.

In FIG. 18, reference 36 indicates a control circuit board forcontrolling a printing operation of the color printer, and reference 38indicates an electrical main power source for electrically energizingthe control circuit board 36.

FIG. 19 shows a part of a schematic block diagram of the control circuitboard 36. As shown in this drawing, the control circuit board 36comprises a printer controller 40 including a microcomputer. The printercontroller 40 receives a series of digital color image-pixel signalsfrom a personal computer or a word processor (not shown) through aninterface circuit (I/F) 42, with each of the digital color image-pixelsignals carrying a digital 2-bit gradation-signal. The received digitalcolor image-pixel signals (i.e., digital cyan image-pixel signalscarrying 2-bit digital gradation signals, digital magenta image-pixelsignals carrying 2-bit digital gradation signals, and digital yellowimage-pixel signals carrying 2-bit digital gradation signals) are oncestored in a memory 44.

Also, the control circuit board 36 is provided with a motor drivercircuit 46 for driving a first set of electric motors 48Y₁ and 48Y₂, asecond set of electric motors 48M₁ and 48M₂ and a third set of electricmotors 48C₁ and 48C₂, which are used to rotationally drive the first setof roller platens 32Y₁ and 32Y₂, the second set of roller platens 32M₁and 32M₂ and the third roller platens 32C₁ and 32C₂, respectively. Eachof the motors 48Y₁ and 48Y₂; 48M₁ and 48M₂; and 48C₁ and 48C₂ is astepping motor, which is driven in accordance with a series of drivepulses outputted from the motor driver circuit 46, the outputting ofdrive pulses from the motor driver circuit 46 to the motors 48Y₁ and48Y₂; 48M₁ and 48M₂; and 48C₁ and 48C₂ being controlled by the printercontroller 40.

During a printing operation, the respective roller platens 32Y₁ and32Y₂; 32M₁ and 32M₂; and 32C₁ and 32C₂ are rotated in a counterclockwisedirection (FIG. 18) by the motors 48Y₁ and 48Y₂; 48M₁ and 48M₂; and 48C₁and 48C₂, with a same peripheral speed. Accordingly, the image-formingsheet 10′, introduced through the entrance opening 22, moves toward theexit opening 24 along the path 26.

Thus, the image-forming sheet 10′ is subjected to pressure rangingbetween the critical compacting-pressure PP₆ and the upper limitpressure PP_(UL) when passing between the thermal head 30Y₁ and theroller platen 32Y₁; to pressure ranging between the criticalcompacting-pressures PP₅ and PP₆ when passing between the thermal head30Y₂ and the roller platen 32Y₂; to pressure ranging between thecritical compacting-pressures PP₄ and PP₅ when passing between thethermal head 30M₁ and the roller platen 32M₁; to pressure rangingbetween the critical compacting-pressures PP₃ and PP₄ when passingbetween the thermal head 30M₂ and the roller platen 32M₂; to pressureranging between the critical compacting-pressures PP₂ and PP₃ whenpassing between the thermal head 30C₁ and the roller platen 32C₁; and topressure ranging between the critical compacting-pressures PP₁ and PP₂when passing between the thermal head 30C₂ and the roller platen 32C₂.

Note, similar to the first embodiment, the introduction of theimage-forming sheet 10′ into the entrance opening 22 of the printer iscarried out such that the transparent protective film sheet 16′ of theimage-forming sheet 10′ comes into contact with the thermal heads 30Y₁and 30Y₂; 30M₁ and 30M₂; and 30C₁ and 30C₂.

In FIG. 19, only one pair of corresponding electric resistance elements,included in each set of thermal heads (30Y₁ and 30Y₂; 30M₁ and 30M₂;30C₁ and 30C₂) is representatively illustrated, and the correspondingpair of electric resistance elements is indicated by references ER₁ andER₂. The respective corresponding electric resistance elements ER₁ andER₂ are selectively and electrically energized by driver circuits 50 ₁and 50 ₂ under control of the printer controller 40.

The driver circuit 50 ₁ includes an AND-gate circuit 52 ₁ and atransistor 54 ₁. As shown in FIG. 19, a set of a strobe signal (STY₁,STM₁ or STC₁) and a control signal (DAY₁, DAM₁ or DAC₁) is inputted fromthe printer controller 40 to two input terminals of the AND-gate circuit52 ₁. A base of the transistor 54 ₁ is connected to an output terminalof the AND-gate circuit 52 ₁; a collector of the transistor 54 ₁ isconnected to an electric power source (V_(cc)); and an emitter of thetransistor 54 ₁ is connected to the electric resistance element ER₁.

Similarly, the driver circuit 50 ₂ includes an AND-gate circuit 52 ₂ anda transistor 54 ₂. As shown in FIG. 19, a set of a strobe signal (STY₂,STM₂ or STC₂) and a control signal (DAY₂, DAM₂ or DAC₂) is inputted fromthe printer controller 40 to two input terminals of the AND-gate circuit52 ₂. A base of the transistor 54 ₂ is connected to an output terminalof the AND-gate circuit 52 ₂; a collector of the transistor 54 ₂ isconnected to an electric power source (V_(cc)); and an emitter of thetransistor 54 ₂ is connected to the electric resistance element ER₂.

When the corresponding pair of electric resistance elements ER₁ and ER₂,as shown in FIG. 19, is one included in the first set of thermal heads30Y₁ and 30Y₂, first, a set of a strobe signal “STY₁” and a controlsignal “DAY₁” is outputted from the printer controller 40, and is theninputted to the input terminals of the AND-gate circuit 52 ₁, during aprinting operation. As shown in a timing chart of FIG. 20, the strobesignal “STY₁” has a pulse width “PWY₁”, and the control signal “DAY₁” isvaried in accordance with binary values of a digital yellow image-pixelsignal “Y” and a 2-bit digital gradation signal “GSY” carried thereby,as shown in a table of FIG. 21.

As is apparent from the table of FIG. 21, if the digital yellowimage-pixel signal “Y” has a value “0”, and if the 2-bit digitalgradation signal “GSY” has a value [00], the control signal “DAY₁” ismaintained at a low-level under control of the printer controller 40(FIGS. 20 and 21). Also, if the digital yellow image-pixel signal “Y”has a value “1”, and if the 2-bit digital gradation signal “GSY” has avalue [10], the control signal “DAY₁” is maintained at a low-level undercontrol of the printer controller 40 (FIGS. 20 and 21).

If the digital yellow image-pixel signal has a value “1”, and if the2-bit digital gradation signal “GSY” has either a value [01] or a value[11], the control signal “DAY₁” is outputted, as a high-level pulse (H)having the same pulse width “PWY₁” as that of the strobe signal “STY₁”,from the printer controller 40. Thus, the electric resistance elementER₁ is electrically energized during a period corresponding to the pulsewidth “PWY₁” of the high-level pulse of the control signal “DAY₁”,whereby the electric resistance element ER₁ is heated to the temperaturebetween the glass-transition temperatures TT₁ and TT₂. Accordingly, in adot area, defined by the electric resistance element ER₁, on theimage-forming sheet 10′, only the microcapsules 18Y₁′ are compacted andbroken, resulting in seepage of the first type of yellow liquid dye Y₁from the compacted and broken microcapsules 18Y₁′.

When the image-forming sheet 10′ has been moved over a given period oftime, i.e. when the dot area defined by the electric resistance elementER₁ has reached a location at which a dot area should be defined on theimage-forming sheet 10′ by the electric resistance element ER₂, a set ofa strobe signal “STY₂” and a control signal “DAY₂” is outputted from theprinter controller 40, and is then inputted to the input terminals ofthe AND-gate circuit 52 ₂. As shown in the timing chart of FIG. 20, thestrobe signal “STY₂” has a pulse width “PWY₂”, being longer than that ofthe strobe signal “STY₁”, and the control signal “DAY₂” is varied inaccordance with binary values of the same digital yellow image-pixelsignal “Y” carrying the 2-bit digital gradation signal “GSY” asmentioned above and shown in the table of FIG. 21.

As is apparent from the table of FIG. 21, if the digital yellowimage-pixel signal “Y” has a value “0”, and if the 2-bit digitalgradation signal “GSY” has a value [00], the control signal “DAY₂” ismaintained at a low-level under control of the printer controller 40(FIGS. 20 and 21). In this case, the dot area, defined by both theelectric resistance elements ER₁ and ER₂, is colored by neither thefirst or second types of yellow dyes Y₁ and Y₂, due to the low-level ofthe control signal “DAY₁” (“Y”=[0] and “GSY”=[00]). Namely, the dot areaconcerned is produced as a white dot on the image-forming sheet 10′.

If the digital yellow image-pixel signal “Y” has a value “1”, and if the2-bit digital gradation signal “GSY” has a value [01], the controlsignal “DAY₂” is maintained at a low-level under control of the printercontroller 40 (FIGS. 20 and 21). In this case, the dot area, defined byboth the electric resistance elements ER₁ and ER₂, is colored by onlythe first type of yellow dye Y₁, as shown in FIG. 22, due to thehigh-level of the control signal “DAY₁” (i.e. “Y”=[1] and “GSY”=[01]).Namely, the dot area concerned is produced as a yellow dot on theimage-forming sheet 10′, and the yellow dot exhibits a first yellowdensity resulting from only the seeped yellow dye Y₁.

If the digital yellow image-pixel signal has a value “1”, and if the2-bit digital gradation signal “GSY” has a value [10], the controlsignal “DAY₂” is outputted, as a high-level pulse (H) having the samepulse width “PWY₂” as that of the strobe signal “STY₂”, from the printercontroller 40. Thus, the electric resistance element ER₂ is electricallyenergized during a period corresponding to the pulse width “PWY₂” of thehigh-level pulse of the control signal “DAY₂”, whereby the electricresistance element ER₂ is heated to the temperature between theglass-transition temperatures TT₂ and TT₃. Thus, in the dot area definedby both the electric resistance elements ER₁ and ER₂, only themicrocapsules 18Y₂′ are compacted and broken, resulting in seepage ofthe second type of yellow liquid dye Y₂ from the compacted and brokenmicrocapsules 18Y₂′. In this case, the dot area, defined by both theelectric resistance elements ER₁ and ER₂, is colored by only the secondtype of yellow dye Y₂, as shown in FIG. 23, due to the low-level of thecontrol signal “DAY₁” (“Y”=[1] and “GSY”=[10]). Namely, the dot areaconcerned is produced as a yellow dot on the image-forming sheet 10′,and the yellow dot exhibits a second yellow density resulting from onlythe seeped yellow dye Y₂.

If the digital yellow image-pixel signal has a value “1”, and if the2-bit digital gradation signal “GSY” has a value [11], the controlsignal “DAY₂” is outputted, as a high-level pulse (H) having the samepulse width “PWY₂” as that of the strobe signal “STY₂”, from the printercontroller 40. Thus, the electric resistance element ER₂ is electricallyenergized during a period corresponding to the pulse width “PWY₂” of thehigh-level pulse of the control signal “DAY₂”, whereby the electricresistance element ER₂ is heated to the temperature between theglass-transition temperatures TT₂ and TT₃. Thus, in the dot area definedby both the electric resistance elements ER₁ and ER₂, the microcapsules18Y₂′ are compacted and broken, resulting in seepage of the second typeof yellow liquid dye Y₂ from the compacted and broken microcapsules18Y₂′. In this case, the dot area, defined by both the electricresistance elements ER₁ and ER₂, is colored by both the first and secondtypes of yellow dyes Y₁ and Y₂, as shown in FIG. 24, due to thehigh-level of the control signal “DAY₁” and “DAY₂” (“Y”=[1] and“GSY”=[11]). Namely, the dot area concerned is produced as a yellow doton the image-forming sheet 10′, and the yellow dot exhibits a thirdyellow density resulting from a mixture of the seeped yellow dyes Y₁ andY₂.

As is apparent from the foregoing, by selectively seeping the first andsecond types of yellow dyes Y₁ and Y₂ from the first and secondmicrocapsules 18Y₁′ and 18Y₂′, it is possible to obtain a variation indensity (gradation) of the yellow dot.

When the corresponding pair of electric resistance elements ER₁ and ER₂,as shown in FIG. 19, is one included in the second set of thermal heads30M₁ and 30M₂, the selective and electrical energization of the electricresistance elements ER₁ and ER₂ is performed in substantially the samemanner as mentioned above and shown in a timing chart of FIG. 25 and atable of FIG. 26, whereby a variation in density (gradation) of themagenta dot is obtainable.

In particular, if a digital magenta image-pixel signal “M” has a value“0”, and if a 2-bit digital gradation signal “GSM” has a value [00], theelectric resistance elements ER₁ and ER₂ cannot be electricallyenergized. Thus, a dot area, defined by both the electric resistanceelements ER₁ and ER₂, is produced as a white dot on the image-formingsheet 10′.

If the digital magenta image-pixel signal “M” has a value “1”, and ifthe 2-bit digital gradation signal “GSM” has a value [01], only theelectric resistance element ER₁ is electrically energized during aperiod corresponding to a pulse width “PWM₁” of a high-level pulse of acontrol signal “DAM₁” which is equal to that of a strobe signal “STM₁”,being longer than that of the strobe signal “STY₂”, whereby the electricresistance element ER₁ is heated to the temperature between theglass-transition temperatures TT₃ and TT₄. On the other hand, theelectric resistance element ER₂ is not electrically energized. Thus, ina dot area, defined by both the electric resistance elements ER₁ andER₂, on the image-forming sheet 10′, only the microcapsules 18M₁′ arecompacted and broken, resulting in seepage of the first type of magentaliquid dye M₁ from the compacted and broken microcapsules 18M₁′. In thiscase, the dot area, defined by both the electric resistance elements ER₁and ER₂, is colored by only the first type of magenta dye M₁. Namely,the dot area concerned is produced as a magenta dot on the image-formingsheet 10′, and the magenta dot exhibits a first magenta densityresulting from only the seeped magenta dye M₁.

If the digital magenta image-pixel signal “M” has a value “1”, and ifthe 2-bit digital gradation signal “GSM” has a value [10], the electricresistance element ER₁ is not electrically energized. On the other hand,the electric resistance element ER₂ is electrically energized during aperiod corresponding to a pulse width “PWM₂” of a high-level pulse of acontrol signal “DAM₂”, which is equal to that of a strobe signal “STM₂”,being longer than that of the strobe signal “STM₁”, whereby the electricresistance element ER₂ is heated to the temperature between theglass-transition temperatures TT₄ and TT₅. Thus, in a dot area definedby both the electric resistance elements ER₁ and ER₂, only themicrocapsules 18M₂′ are compacted and broken, resulting in seepage ofthe second type of magenta liquid dye M₂ from the compacted and brokenmicrocapsules 18M₂′. In this case, a dot area, defined by both theelectric resistance elements ER₁ and ER₂, is colored by only the secondtype of magenta dye M₂. Namely, the dot area concerned is produced as amagenta dot on the image-forming sheet 10′, and the magenta dot exhibitsa second magenta density resulting from only the seeped magenta dye M₂.

If the digital magenta image-pixel signal “M” has a value “1”, and ifthe 2-bit digital gradation signal “GSM” has a value [11], electricalenergization of both the electric resistance elements ER₁ and ER₂ iscarried out. Thus, in a dot area defined by both the electric resistanceelements ER₁ and ER₂, both the microcapsules 18M₁′ and 18M₂′ arecompacted and broken, resulting in seepage of the first and second typesof magenta liquid dyes M₁ and M₂ from the compacted and brokenmicrocapsules 18M₁′ and 18M₂′. In this case, the dot area, defined byboth the electric resistance elements ER₁ and ER₂, is colored by boththe first and second types of magenta dyes M₁ and M₂. Namely, the dotarea concerned is produced as a magenta dot on the image-forming sheet10′, and the magenta dot exhibits a third magenta density resulting froma mixture of the seeped magenta dyes M₁ and M₂.

Further, when the corresponding pair of electric resistance elements ER₁and ER₂, as shown in FIG. 19, is one included in the third set ofthermal heads 30C₁ and 30C₂, the selective and electrical energizationof the electric resistance elements ER₁ and ER₂ is performed insubstantially the same manner as mentioned above and shown in a timingchart of FIG. 27 and a table of FIG. 28, whereby a variation in density(gradation) of the cyan dot is obtainable.

In particular, if a digital cyan image-pixel signal “C” has a value “0”,and if a 2-bit digital gradation signal “GSC” has a value [00], theelectric resistance elements ER₁ and ER₂ is not electrically energized.Thus, a dot area, defined by both the electric resistance elements ER₁and ER₂, is produced as a white dot on the image-forming sheet 10′.

If the digital cyan image-pixel signal “C” has a value “1”, and if the2-bit digital gradation signal “GSC” has a value [01], only the electricresistance element ER₁ is electrically energized during a periodcorresponding to a pulse width “PWC₁” of a high-level pulse of a controlsignal “DAC₁”, which is equal to that of a strobe signal “STC₁”, beinglonger than that of the strobe signal “STM₂”, whereby the electricresistance element ER₁ is heated to the temperature between theglass-transition temperatures TT₅ and TT₆. On the other hand, theelectric resistance element ER₂ is not electrically energized. Thus, ina dot area, defined by both the electric resistance elements ER₁ andER₂, on the image-forming sheet 10′, only the microcapsules 18C₁′ arecompacted and broken, resulting in seepage of the first type of cyanliquid dye C₁ from the compacted and broken microcapsules 18C₁′. In thiscase, a dot area, defined by both the electric resistance elements ER₁and ER₂, is colored by only the first type of cyan dye C₁. Namely, thedot area concerned is produced as a cyan dot on the image-forming sheet10′, and the cyan dot exhibits a first cyan density resulting from onlythe seeped cyan dye C₁.

If the digital cyan image-pixel signal “C” has a value “1”, and if the2-bit digital gradation signal “GSC” has a value [10], the electricresistance element ER₁ is not electrically energized. On the other hand,the electric resistance element ER₂ is electrically energized during aperiod corresponding to a pulse width “PWC₂” of a high-level pulse of acontrol signal “DAC₂”, which is equal to that of a strobe signal “STC₂”,being longer than that of the strobe signal “STC₁”, whereby the electricresistance element ER₂ is heated to the temperature between theglass-transition temperature TT₆ and the upper limit temperatureTT_(UL). Thus, in a dot area defined by both the electric resistanceelements ER₁ and ER₂, only the microcapsules 18C₂′ are compacted andbroken, resulting in seepage of the second type of cyan liquid dye C₂from the compacted and broken microcapsules 18C₂′. In this case, a dotarea, defined by both the electric resistance elements ER₁ and ER₂, iscolored by only the second type of cyan dye C₂. Namely, the dot areaconcerned is produced as a cyan dot on the image-forming sheet 10′, andthe cyan dot exhibits a second cyan density resulting from only theseeped cyan dye C₂.

If the digital cyan image-pixel signal “C” has a value “1”, and if the2-bit digital gradation signal “GSC” has a value [11], electricalenergization of both the electric resistance elements ER₁ and ER₂ iscarried out. Thus, in a dot area defined by both the electric resistanceelements ER₁ and ER₂, both the microcapsules 18C₁′ and 18C₂′ arecompacted and broken, resulting in seepage of the first and second typesof cyan liquid dyes C₁ and C₂ from the compacted and brokenmicrocapsules 18C₁′ and 18C₂′. In this case, the dot area, defined byboth the electric resistance elements ER₁ and ER₂, is colored by boththe first and second types of cyan dyes C₁ and C₂. Namely, the dot areaconcerned is produced as a cyan dot on the image-forming sheet 10′, andthe cyan dot exhibits a third cyan density resulting from a mixture ofthe seeped cyan dyes C₁ and C₂.

Note, similar to the first embodiment, the yellow dot, the magenta dotand the cyan dot, produced on the image-forming sheet 10′, also have adot size of about 50 μm to about 100 μm, and the first, second, third,fourth, fifth and sixth types of microcapsules 18Y₁′, 18Y₂′, 18M₁′,18M₂′, 18C₁′and 18C₂′ are uniformly included in a dot area to beproduced on the image-forming sheet 10′.

With reference to FIG. 29, a modification of the first and secondembodiments of the image-forming sheets 10 and 10′ is shown. In thismodified embodiment, an additional layer of microcapsules 18B is furtherformed over the microcapsules layer 14 (14′), and each of themicrocapsules 18B is filled with black liquid dye. Of course, the blackmicrocapsules 18B may be produced by the same polymerization method asmentioned previously, and may have an average diameter of severalmicrons, for example, 5 μm to 10 μm. Also, the formation of a blackmicrocapsule layer is performed in a similar manner to the formation ofthe microcapsule layer 14 (14′).

A microcapsule wall of each black microcapsule 18B may be formed of asuitable resin material, usually colored white, which exhibits acharacteristic temperature such that it is thermally fused orplasticized when being heated to more than the upper limit temperatureT_(UL) or TT_(UL), as shown in the graphs of FIGS. 4 and 17.

Before a color image can be formed on the image-forming sheet featuringthe layer of black microcapsules 18B, a color printer, as shown in FIGS.5 and 18, must be provided with an additional set of a line thermal headand a roller platen. The additional line thermal head is constitutedsuch that an electric resistance element thereof is heated to more thanthe upper limit temperature T_(UL) (TT_(UL)), and the additional rollerplaten is elastically pressed against the additional line thermal headat a pressure less than the pressure P₁ (PP₁), as shown in the graphs ofFIGS. 4 and 17. When a black dot should be produced on the image-formingsheet featuring the layer of black microcapsules 18B, a correspondingelectric resistance element of the additional line thermal head iselectrically energized so as to be heated to the temperature beyond theupper limit temperature T_(UL) (TT_(UL)).

As is well known, it is possible to produce black by mixing the threeprimary-colors: cyan, magenta and yellow, but, in reality, it isdifficult to generate a true or vivid black by the mixing of the primarycolors. Nevertheless, by using the image-forming sheet featuring thelayer of black microcapsules 18B, it is possible to produce the true orvivid black.

Although in the embodiment above, the back microcapsules 18B is formedover an additional layer over the underlying colored capsule layer 14 or14′, these black microcaspules may also be distributed in layer 14 or14′, either in addition to, or as an alternative, to the single blacklayer.

In the first and second embodiments, six types of monochromatic dyes(for example, black or gray), having differing densities, may beencapsulated in the first, second, third, fourth, fifth and sixth typesof microcapsules (18Y₁ or 18Y₁′, 18Y₂ or 18Y₂′, 18M₁ or 18M₁′, 18M₂ or18M₂′, 18C₁ or 18C₁′, 18C₂ or 18C₂′). In this case, of course, it ispossible to produce a monochromatic dot with various differing densities(gradations).

For a dye to be encapsulated in the microcapsules (18Y₁ or 18Y₁′, 18Y₂or 18Y₂′, 18M₁ or 18M₁′, 18M₂ or 18M₂′, 18C₁ or 18C₁′, 18C₂ or 18C₂′),leuco-pigment may be utilized. As is well-known, the leuco-pigment perse exhibits no color. Accordingly, in this case, color developer iscontained in the binder, which forms a part of the layer ofmicrocapsules (14, 14′).

Also, a wax-type ink may be utilized for an ink to be encapsulated inthe microcapsules (18Y₁ or 18Y₁′, 18Y₂ or 18Y₂′, 18M₁ or 18M₁′, 18M₂ or18M₂′, 18C₁ or 18C₁′, 18C₂ or 18C₂′). In this case, the wax-type inkshould thermally fuse at a temperature lower than a lowest criticaltemperature, as indicated by references T₁ and TT₁, shown in FIGS. 4 and17, respectively.

With the enlightenment of the teachings of the present application, oneof ordinary skill could select appropriate wall thickness for themicrocapsules from the disclosed preferable range of 5-10 μm whichshatter at the temperatures and pressureses as described herein.

Finally, it will be understood by those skilled in the art that theforegoing description is of preferred embodiments of the image-formingsubstrate, and that various changes and modifications may be made to thepresent invention without departing from the spirit and scope thereof.

By way of non-limiting example, although the present applicationdescribes the microcapsules in terms of prefered color and/or differenttypes of capsules, the invention is not so limited. The use of anynumber and/or type of colors, and the mechanism necessary to shattersuch number of different capsules, fall within the teachings of thepresent invention.

In another example, the prefered embodiment of the present applicationcontemplates the uses of multiple colors uniformly distributed in asingle layer, either alone or with an additional black/grey layerthereon. The invention is not, however, so limited. The number, type,and color of microcapsules may be provided in any number of desiredlayers. By way of example, the colors could be applied in layers, i.e.,one specific color to a layer. In an alternative, any combination of oneor more colors could appear in different layers. In a furtheralternative, indentical color combinations could be present in seperatelayers (such as to increase density). Such combinations representvariations of the teachings herein, and thus fall within the scope andspirit of the invention.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 10-69452 (filed on Mar. 4, 1998) which isexpressly incorporated herein, by reference, in its entirety.

What is claimed:
 1. An image-forming substrate comprising: a basemember; and a layer of microcapsules, coated over said base member,containing a first type of microcapsule filled with a first type offirst-single-color dye, and a second type of microcapsule filled with asecond type of first-single-color dye, said first type of microcapsuleexhibiting a first temperature/pressure characteristic such that, whensaid first type of microcapsule is squashed under a first predeterminedpressure at a first predetermined temperature, discharge of said firsttype of first-single-color dye from said squashed microcapsule occurs,said second type of microcapsule exhibiting a secondtemperature/pressure characteristic such that, when said second type ofmicrocapsule is squashed under said first predetermined pressure at asecond predetermined temperature, discharge of said second type offirst-single-color dye from said squashed microcapsule occurs.
 2. Animage-forming substrate as set forth in claim 1, wherein said first typeof first-single-color dye exhibits a same density as that of said secondtype of first-single-color dye.
 3. An image-forming substrate as setforth in claim 1, wherein said first type of first-single-color dyeexhibits a density different from that of said second type offirst-single-color dye.
 4. An image-forming substrate as set forth inclaim 1, wherein said layer of microcapsules further comprises a thirdtype of microcapsule filled with a first type of second-single-colordye, and a fourth type of microcapsule filled with a second type ofsecond-single-color dye, said third type of microcapsule exhibiting athird temperature/pressure characteristic such that, when said thirdtype of microcapsule is squashed under a second predetermined pressureat a third predetermined temperature, discharge of said first type ofsecond-single-color dye from said squashed microcapsule occurs, saidfourth type of microcapsule exhibiting a fourth temperature/pressurecharacteristic such that, when said fourth type of microcapsule issquashed under said second predetermined pressure at a fourthpredetermined temperature, discharge of said second type ofsecond-single-color dye from said squashed microcapsule occurs.
 5. Animage-forming substrate as set forth in claim 4, wherein said first typeof second-single-color dye exhibits a same density as that of saidsecond type of second-single-color dye.
 6. An image-forming substrate asset forth in claim 4, wherein said first type of second-single-color dyeexhibits a density different from that of said second type ofsecond-single-color dye.
 7. An image-forming substrate as set forth inclaim 4, wherein said layer of microcapsules further comprises a fifthtype of microcapsule filled with a first type of third-single-color dye,and a sixth type of microcapsule filled with a second type ofthird-single-color dye, said fifth type of microcapsule exhibiting afifth temperature/pressure characteristic such that, when said fifthtype of microcapsule is squashed under a third predetermined pressure ata fifth predetermined temperature, discharge of said first type ofthird-single-color dye from said squashed microcapsule occurs, saidsixth type of microcapsule exhibiting a sixth temperature/pressurecharacteristic such that, when said sixth type of microcapsule issquashed under said third predetermined pressure at a sixthpredetermined temperature, discharge of said second type ofthird-single-color dye from said squashed microcapsule occurs.
 8. Animage-forming substrate as set forth in claim 7, wherein said first typeof third-single-color dye exhibits a same density as that of said secondtype of third-single-color dye.
 9. An image-forming substrate as setforth in claim 7, wherein said first type of third-single-color dyeexhibits a density different from that of said second type ofthird-single-color dye.
 10. An image-forming substrate as set forth inclaim 7, wherein said first-single-color dye, said second-single-colordye and said third-single-color dye comprise three-primary color dyes.11. An image-forming substrate as set forth in claim 10, furthercomprising an additional layer of microcapsules filled with black dyecoated over said layer of microcapsules, said microcapsules included insaid additional layer of microcapsules being formed of resin such thatthey are thermally plasticized at a greater temperature than said sixthpredetermined temperature and under a lower pressure than said thirdpredetermined pressure.
 12. An image-forming substrate as set forth inclaim 7, wherein said first-single-color dye, said second-single-colordye and said third-single-color dye comprise a same single-color dyeexhibiting differing densities.
 13. An image-forming apparatus usingsaid image-forming substrate as set forth in claim 7, which comprises: afirst pressure applicator that exerts said first predetermined pressureon said image-forming substrate; a second pressure applicator thatexerts said second predetermined pressure on said image-formingsubstrate; a third pressure applicator that exerts said thirdpredetermined pressure on said image-forming substrate; a first thermalheater that selectively heats a localized area, on which said firstpredetermined pressure is exerted by said first pressure applicator, toone of said first predetermined temperature and said secondpredetermined temperature in accordance with first-single-colorimage-pixel information carrying gradation information; a second thermalheater that selectively heats a localized area, on which said secondpredetermined pressure is exerted by said second pressure applicator, toone of said third predetermined temperature and said fourthpredetermined temperature in accordance with second-single-colorimage-pixel information carrying gradation information; and a thirdthermal heater that selectively heats a localized area, on which saidthird predetermined pressure is exerted by said third pressureapplicator, to one of said fifth predetermined temperature and saidsixth predetermined temperature in accordance with third-single-colorimage-pixel information carrying gradation information.
 14. Animage-forming apparatus using said image-forming substrate as set forthin claim 4, which comprises: a first pressure applicator that exertssaid first predetermined pressure on said image-forming substrate; asecond pressure applicator that exerts said second predeterminedpressure on said image-forming substrate; a first thermal heater thatselectively heats a localized area, on which said first predeterminedpressure is exerted by said first pressure applicator, to one of saidfirst predetermined temperature and said second predeterminedtemperature in accordance with first-single-color image-pixelinformation carrying gradation information; and a second thermal heaterthat selectively heats a localized area, on which said secondpredetermined pressure is exerted by said second pressure applicator, toone of said third predetermined temperature and said fourthpredetermined temperature in accordance with second-single-colorimage-pixel information carrying gradation information.
 15. Animage-forming apparatus using said image-forming substrate as set forthin claim 1, which comprises: a pressure applicator that exerts saidfirst predetermined pressure on said image-forming substrate; and athermal heater that selectively heats a localized area, on which saidfirst predetermined pressure is exerted by said first pressureapplicator, to one of said first predetermined temperature and saidsecond predetermined temperature in accordance with image-pixelinformation carrying gradation information.
 16. An image-formingsubstrate comprising: a base member; and a layer of microcapsules,coated over said base member, containing a first type of microcapsulefilled with a first type of first-single-color dye, and a second type ofmicrocapsule filled with a first type of first-single-color dye, saidfirst type of microcapsule exhibiting a first temperature/pressurecharacteristic such that, when said first type of microcapsule issquashed under a first predetermined pressure at a first predeterminedtemperature, discharge of said first type of first-single-color dye fromsaid squashed microcapsule occurs, said second type of microcapsuleexhibiting a second temperature/pressure characteristic such that, whensaid second type of microcapsule is squashed under a secondpredetermined pressure at a second predetermined temperature, dischargeof said second type of first-single-color dye from said squashedmicrocapsule occurs.
 17. An image-forming substrate as set forth inclaim 16, wherein said layer of microcapsules further comprises a thirdtype of microcapsule filled with a first type of second-single-colordye, and a fourth type of microcapsule filled with a second type ofsecond-single-color dye, said third type of microcapsule exhibiting athird temperature/pressure characteristic such that, when said thirdtype of microcapsule is squashed under a third predetermined pressure ata third predetermined temperature, discharge of said first type ofsecond-single-color dye from said squashed microcapsule occurs, saidfourth type of microcapsule exhibiting a fourth temperature/pressurecharacteristic such that, when said fourth type of microcapsule issquashed under a fourth predetermined pressure at a fourth predeterminedtemperature, discharge of said second type of second-single-color dyefrom said squashed microcapsule occurs.
 18. An image-forming substrateas set forth in claim 17, wherein said layer of microcapsules furthercomprises a fifth type of microcapsule filled with a first type ofthird-single-color dye, and a sixth type of microcapsule filled with asecond type of third-single-color dye, said fifth type of microcapsuleexhibiting a fifth temperature/pressure characteristic such that, whensaid fifth type of microcapsule is squashed under a fifth predeterminedpressure at a fifth predetermined temperature, discharge of said firsttype of third-single-color dye from said squashed microcapsule occurs,said sixth type of microcapsule exhibiting a sixth temperature/pressurecharacteristic such that, when said sixth type of microcapsule issquashed under a sixth predetermined pressure at a sixth predeterminedtemperature, discharge of said second type of third-single-color dyefrom said squashed microcapsule occurs.
 19. An image-forming substrateas set forth in claim 18, wherein said first-single-color dye, saidsecond-single-color dye and said third-single-color dye comprisesthree-primary color dyes.
 20. An image-forming substrate as set forth inclaim 19, further comprising an additional layer of microcapsules filledwith black dye coated over said layer of microcapsules, saidmicrocapsules included in said additional layer of microcapsules beingformed of resin such that they are thermally plasticized at a greatertemperature than said sixth predetermined temperature and under a lowerpressure than said sixth predetermined pressure.
 21. An image-formingsubstrate as set forth in claim 18, wherein said first-single-color dye,said second-single-color dye and said third-single-color dye comprise asame single-color dye exhibiting differing densities.
 22. Animage-forming apparatus using said image-forming substrate as set forthin claim 18, which comprises: a first pressure applicator that exertssaid first predetermined pressure on said image-forming substrate; asecond pressure applicator that exerts said second predeterminedpressure on said image-forming substrate; a third pressure applicatorthat exerts said third predetermined pressure on said image-formingsubstrate; a fourth pressure applicator that exerts said fourthpredetermined pressure said image-forming substrate; a fifth pressureapplicator that exerts said fifth predetermined pressure on saidimage-forming substrate; a sixth pressure applicator that exerts saidsixth predetermined pressure on said image-forming substrate; a firstthermal heater that selectively heats a localized area, on which saidfirst predetermined pressure is exerted by said pressure applicator, tosaid first predetermined temperature in accordance withfirst-single-color image-pixel information carrying gradationinformation; a second thermal heater that selectively heats a localizedarea, on which said second predetermined pressure is exerted by saidsecond pressure applicator, to said second predetermined temperature inaccordance with said first-single-color image-pixel information carryingsaid gradation information; a third thermal heater that selectivelyheats a localized area, on which said third predetermined pressure isexerted by said third pressure applicator, to said third predeterminedtemperature in accordance with second-single-color image-pixelinformation carrying gradation information; a fourth thermal heater thatselectively heats a localized area, on which said fourth predeterminedpressure is exerted by said fourth pressure applicator, to said fourthpredetermined temperature in accordance with said second-single-colorimage-pixel information carrying said gradation information; a fifththermal heater that selectively heats a localized area, on which saidfifth predetermined pressure is exerted by said fifth pressureapplicator, to said fifth predetermined temperature in accordance withthird-single-color image-pixel information carrying gradationinformation; and a sixth thermal heater that selectively heats alocalized area, on which said sixth predetermined pressure is exerted bysaid sixth pressure applicator, to said sixth predetermined temperaturein accordance with said third-single-color image-pixel informationcarrying said gradation information.
 23. An image-forming apparatususing said image-forming substrate as set forth in claim 17, whichcomprises: a first pressure applicator that exerts said firstpredetermined pressure on said image-forming substrate; a secondpressure applicator that exerts said second predetermined pressure onsaid image-forming substrate; a third pressure applicator that exertssaid third predetermined pressure on said image-forming substrate; afourth pressure applicator that exerts said fourth predeterminedpressure on said image-forming substrate; a first thermal heater thatselectively heats a localized area, on which said first predeterminedpressure is exerted by said first pressure applicator, to said firstpredetermined temperature in accordance with first-single-colorimage-pixel information carrying gradation information; a second thermalheater that selectively heats a localized area, on which said secondpredetermined pressure is exerted by said second pressure applicator, tosaid second predetermined temperature in accordance with saidfirst-single-color image-pixel information carrying said gradationinformation; a third thermal heater that selectively heats a localizedarea, on which said third predetermined pressure is exerted by saidthird pressure applicator, to said third predetermined temperature inaccordance with second-single-color image-pixel information carryinggradation information; and a fourth thermal heater that selectivelyheats a localized area, on which said fourth predetermined pressure isexerted by said fourth pressure applicator, to said fourth predeterminedtemperature in accordance with said second-single-color image-pixelinformation carrying said gradation information.
 24. An image-formingapparatus using said image-forming substrate as set forth in claim 16,which comprises: a first pressure applicator that exerts said firstpredetermined pressure on said image-forming substrate; a secondpressure applicator that exerts said second predetermined pressure onsaid image-forming substrate; a first thermal heater that selectivelyheats a localized area, on which said first predetermined pressure isexerted by said first pressure applicator, to said first predeterminedtemperature in accordance with first-single-color image-pixelinformation carrying gradation information; and a second thermal heaterthat selectively heats a localized area, on which said secondpredetermined pressure is exerted by said second pressure applicator, tosaid second predetermined temperature in accordance with saidfirst-single-color image-pixel information carrying said gradationinformation.